gRPC client API improvements (2.0)

[aseovic]

This PR has two related things

1. The same set of client API improvements that make it easier to use as in gRPC client API improvements (1.x) #1850
2. Renamed @RpcService, @RpcMethod, and @GrpcServiceProxy annotations to @Grpc, @GrpcMethod and @GrpcProxy, to make them consistent with other annotations and/or less verbose.

Current annotations just don't look right when combined:

       @RpcService(name = "FooService")
       @GrpcChannel(name = "foo")
       public interface FooServiceClient {
           ...
       };

This PR changes the above to:

       @Grpc(name = "FooService")
       @GrpcChannel(name = "foo")
       public interface FooServiceClient {
           ...
       };

which reads better and makes a lot more sense long term.

The new name also better describes what the thing is -- it is a gRPC service, not a generic RPC service.

This is a breaking change from 1.x, but considering that gRPC is experimental in 1.x and that 2.0 is a major release, this seems like the right time to make this change.

[spericas]

LGTM

[tomas-langer]

Merged changelog.md manually.