Remove Subchannel.State and track state inside load balancer

[JamesNK]

A load balancer shouldn't use Subchannel.State because it can be updated from other threads while a load balancer is building the picker. Instead, a load balancer should track its own copy of states.

Public API change is ok because this isn't final yet.

[JamesNK]

Noticed in test failure below. Load balancer creates picker with two ready subchannels even though it has only been notified about one ready subchannel.

  Failed UnaryCall_UnavailableAddress_FallbackToWorkingAddress [73 ms]
  Error Message:
     Expected: some item equal to "127.0.0.1:50051"
  But was:  < "127.0.0.1:50052", "127.0.0.1:50052" >

  Stack Trace:
     at Grpc.AspNetCore.FunctionalTests.Balancer.RoundRobinBalancerTests.UnaryCall_UnavailableAddress_FallbackToWorkingAddress() in /home/runner/work/grpc-dotnet/grpc-dotnet/test/FunctionalTests/Balancer/RoundRobinBalancerTests.cs:line 269
   at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.BlockUntilCompleted()
   at NUnit.Framework.Internal.MessagePumpStrategy.NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter awaiter)
   at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(Func`1 invoke)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.RunTestMethod(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.<>c__DisplayClass1_0.<Execute>b__0()
   at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)

  Standard Output Messages:
 0.000s GrpcTestContext - Information: Starting RoundRobinBalancerTests.UnaryCall_UnavailableAddress_FallbackToWorkingAddress
 0.027s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '1' created with addresses: 127.0.0.1:50051
 0.027s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' created with addresses: 127.0.0.1:50052
 0.027s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '1' connection requested.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '1' state changed to Connecting. Detail: 'Connection requested.'.
 0.027s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '1' state changed to Connecting. Detail: 'Connection requested.'.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel state updated to Connecting.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '1' connecting to transport.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '2' connection requested.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' state changed to Connecting. Detail: 'Connection requested.'.
 0.027s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '2' state changed to Connecting. Detail: 'Connection requested.'.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '2' connecting to transport.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel state updated to Ready.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.027s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' state changed to Ready. Detail: 'Successfully connected to socket.'.
 0.027s GrpcTestContext - Information: Waiting for subchannel ready count: 2
 0.027s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '2' state changed to Ready. Detail: 'Successfully connected to socket.'.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.027s GrpcTestContext - Information: Current subchannel ready count: 2
 0.027s GrpcTestContext - Information: Ready subchannel: Id: 1, Addresses: 127.0.0.1:50051, State: Ready, Current address: 127.0.0.1:50051
 0.027s GrpcTestContext - Information: Ready subchannel: Id: 2, Addresses: 127.0.0.1:50052, State: Ready, Current address: 127.0.0.1:50052
 0.027s GrpcTestContext - Information: Finished waiting for subchannel ready.
 0.027s Grpc.Net.Client.Internal.GrpcCall - Debug: Starting gRPC call. Method type: 'Unary', URI: 'http://loadbalancer.temporary.invalid/DynamicService/UnaryMethod'.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Trace: Pick started.
 0.027s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Successfully picked subchannel id '2' with address 127.0.0.1:50052.
 0.027s Grpc.Net.Client.Internal.GrpcCall - Debug: Sending message.
 0.027s Grpc.Net.Client.Internal.GrpcCall - Trace: Serialized 'Greet.HelloRequest' to 11 byte message.
 0.028s Grpc.Net.Client.Internal.GrpcCall - Trace: Message sent.
 0.028s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '1' state changed to Ready. Detail: 'Successfully connected to socket.'.
 0.028s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '1' state changed to Ready. Detail: 'Successfully connected to socket.'.
 0.028s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Trace: Response headers received.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Debug: Reading message.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Trace: Deserializing 11 byte message to 'Greet.HelloReply'.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Trace: Received message.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Debug: Finished gRPC call.
 0.029s Grpc.Net.Client.Internal.GrpcCall - Debug: Starting gRPC call. Method type: 'Unary', URI: 'http://loadbalancer.temporary.invalid/DynamicService/UnaryMethod'.
 0.029s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Trace: Pick started.
 0.029s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Successfully picked subchannel id '2' with address 127.0.0.1:50052.
 0.030s Grpc.Net.Client.Internal.GrpcCall - Debug: Sending message.
 0.030s Grpc.Net.Client.Internal.GrpcCall - Trace: Serialized 'Greet.HelloRequest' to 11 byte message.
 0.030s Grpc.Net.Client.Internal.GrpcCall - Trace: Message sent.
 0.036s Grpc.Net.Client.Internal.GrpcCall - Trace: Response headers received.
 0.036s Grpc.Net.Client.Internal.GrpcCall - Debug: Reading message.
 0.036s Grpc.Net.Client.Internal.GrpcCall - Trace: Deserializing 11 byte message to 'Greet.HelloReply'.
 0.036s Grpc.Net.Client.Internal.GrpcCall - Trace: Received message.
 0.036s Grpc.Net.Client.Internal.GrpcCall - Debug: Finished gRPC call.
 0.070s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' state changed to Idle. Detail: 'Disconnected.'.
 0.070s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '2' state changed to Idle. Detail: 'Disconnected.'.
 0.070s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.070s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Refreshing resolver because subchannel id '2' is in state Idle.
 0.070s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Connections unchanged.
 0.070s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Requesting connection for subchannel id '2' because it is in state Idle.
 0.070s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '2' connection requested.
 0.070s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' state changed to Connecting. Detail: 'Connection requested.'.
 0.070s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '2' state changed to Connecting. Detail: 'Connection requested.'.
 0.070s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.070s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '2' connecting to transport.
 0.071s Grpc.Net.Client.Balancer.Subchannel - Debug: Subchannel id '2' state changed to TransientFailure. Detail: 'Error connecting to subchannel.'.
 System.Net.Sockets.SocketException (111): Connection refused
    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
    at System.Net.Sockets.Socket.<ConnectAsync>g__WaitForConnectWithCancellation|277_0(AwaitableSocketAsyncEventArgs saea, ValueTask connectTask, CancellationToken cancellationToken)
    at Grpc.Net.Client.Balancer.Internal.SocketConnectivitySubchannelTransport.TryConnectAsync(CancellationToken cancellationToken) in /home/runner/work/grpc-dotnet/grpc-dotnet/src/Grpc.Net.Client/Balancer/Internal/SocketConnectivitySubchannelTransport.cs:line 138
 0.071s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Processing subchannel id '2' state changed to TransientFailure. Detail: 'Error connecting to subchannel.'.
 System.Net.Sockets.SocketException (111): Connection refused
    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)
    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
    at System.Net.Sockets.Socket.<ConnectAsync>g__WaitForConnectWithCancellation|277_0(AwaitableSocketAsyncEventArgs saea, ValueTask connectTask, CancellationToken cancellationToken)
    at Grpc.Net.Client.Balancer.Internal.SocketConnectivitySubchannelTransport.TryConnectAsync(CancellationToken cancellationToken) in /home/runner/work/grpc-dotnet/grpc-dotnet/src/Grpc.Net.Client/Balancer/Internal/SocketConnectivitySubchannelTransport.cs:line 138
 0.071s Grpc.Net.Client.Balancer.Internal.ConnectionManager - Debug: Channel picker updated.
 0.071s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Refreshing resolver because subchannel id '2' is in state TransientFailure.
 0.071s Grpc.Net.Client.Balancer.RoundRobinBalancer - Trace: Connections unchanged.
 0.071s Grpc.Net.Client.Balancer.Subchannel - Trace: Subchannel id '2' starting connect backoff of 00:00:01.
 0.073s GrpcTestContext - Information: Finishing RoundRobinBalancerTests.UnaryCall_UnavailableAddress_FallbackToWorkingAddress