Fixes race condition in EventStream

[ekcasey]

Locks the mutex for reading when getting Length(). I am hoping this will solve the following data race go test is reporting:

WARNING: DATA RACE
Read at 0x00c000224a30 by goroutine 5563:
  sync/atomic.LoadInt32()
      /opt/hostedtoolcache/go/1.20.6/x64/src/runtime/race_amd64.s:202 +0xb
  sync/atomic.LoadInt32()
      <autogenerated>:1 +0x16
  github.com/asynkron/protoactor-go/actor.(*deadLetterProcess).SendSystemMessage()
      /home/runner/go/pkg/mod/github.com/asynkron/protoactor-go@v0.0.0-20230703103118-df5e4f42621c/actor/deadletter.go:97 +0x18c
  github.com/asynkron/protoactor-go/actor.(*deadLetterProcess).Stop()
      /home/runner/go/pkg/mod/github.com/asynkron/protoactor-go@v0.0.0-20230703103118-df5e4f42621c/actor/deadletter.go:104 +0x2c
  github.com/asynkron/protoactor-go/actor.(*actorContext).Stop()
      /home/runner/go/pkg/mod/github.com/asynkron/protoactor-go@v0.0.0-20230703103118-df5e4f42621c/actor/actor_context.go:436 +0x298
...

Previous write at 0x00c000224a30 by goroutine 5570:
  github.com/asynkron/protoactor-go/eventstream.(*EventStream).Subscribe()
      /home/runner/go/pkg/mod/github.com/asynkron/protoactor-go@v0.0.0-20230703103118-df5e4f42621c/eventstream/eventstream.go:44 +0x1e4
...

I am assuming the atomic read was supposed to protect against? But if the write isn't atomic I think there is still the potential for issues. As I understand it read and write should either both use the mutex (as in this PR) or ensure that all operations are atomic.

[CLAassistant]

All committers have signed the CLA.

[rogeralsing]

Awesome!