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 toRawString and toRawStringWithAddress in serialization
* Replaced var uid in ActorCell and ChildRestartStats with
  constructor parameters (path)
* Create the uid in one single place, in makeChild in parent
Changed DeathWatch, see #3072
Optimize ActorPath.toString and incorparate feedback, see #3072
Update documentation and migration guide, see #3072

* toSerializationFormat
Optimized toStringWithAddress also, see #3070
Cleanup docs, see #3072