RemoteWatcher race-condition fix

[zbynek001]

There is a race condition in the RemoteWatcher when Unwatching shortly before receiving Terminated
resulting in:

System.Collections.Generic.KeyNotFoundException: The given key '[akka.tcp://engine@10.0.0.5:30002/system/XXXXXXXXXX#399075042]' was not present in the dictionary.
 at TValue System.Collections.Generic.Dictionary<TKey, TValue>.get_Item(TKey key)
 at void Akka.Remote.RemoteWatcher.ProcessTerminated(IInternalActorRef watchee, bool existenceConfirmed, bool addressTerminated)
 at void Akka.Cluster.ClusterRemoteWatcher.OnReceive(object message)
 at bool Akka.Actor.UntypedActor.Receive(object message)
 at bool Akka.Actor.ActorBase.AroundReceive(Receive receive, object message)
 at void Akka.Actor.ActorCell.ReceiveMessage(object message)
 at void Akka.Actor.ActorCell.AutoReceiveMessage(Envelope envelope)
 at void Akka.Actor.ActorCell.Invoke(Envelope envelope)

[Aaronontheweb]

@zbynek001 I think you've earned an Akka.NET tshirt or something by now with all of these fixes.

[Aaronontheweb]

Yep, looks like a definite bug on our part.

[zbynek001]

@Aaronontheweb I'm happy to help, since I'm using it ;)