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