Add UID to RemoteActorRef, see #3072

* Sending to a previous incarnation of an actor shall fail,
  to make remote actors work the same way as local ones (in
  the sense that after Terminated() the ref is not working anymore)
* Changed equality of ActorRef to take the uid into account
* Parse uid fragment in RelativeActorPath and ActorPathExtractor
* Handle uid in getChild and in RemoteSystemDaemon
* Use toSerializationFormat and toSerializationFormatWithAddress
  in serialization
* Replaced var uid in ActorCell and ChildRestartStats with
  constructor parameters (path)
* Create the uid in one single place, in makeChild in parent
* Handle ActorRef with and without uid in DeathWatch
* Optimize ActorPath.toString and friends
* Update documentation and migration guide

akka-remote/src/main/scala/akka/remote/serialization/ProtobufSerializer.scala

@@ -19,8 +19,8 @@ object ProtobufSerializer {
    */
   def serializeActorRef(ref: ActorRef): ActorRefProtocol = {
     val identifier: String = Serialization.currentTransportAddress.value match {
-      case null    ⇒ ref.path.toString
-      case address ⇒ ref.path.toStringWithAddress(address)
+      case null    ⇒ ref.path.toSerializationFormat
+      case address ⇒ ref.path.toSerializationFormatWithAddress(address)
     }
     ActorRefProtocol.newBuilder.setPath(identifier).build
   }