Disallow re-joining, see #2873

* Disallow join requests when already part of a cluster
* Remove wipe state when joining, since join can only be
  performed from empty state
* When trying to join, only accept gossip from that member
* Ignore gossips from unknown (and unreachable) members
* Make sure received gossip contains selfAddress
* Test join of fresh node with same host:port
* Remove JoinTwoClustersSpec
* Welcome message as reply to Join
* Retry unsucessful join request
* AddressUidExtension
* Uid in cluster Member identifier
  To be able to distinguish nodes with same host:port
  after restart.
* Ignore gossip with wrong uid
* Renamed Remove command to Shutdown
* Use uid in vclock identifier
* Update sample, Member apply is private
* Disabled config duration syntax and cleanup of io settings
* Update documentation

akka-remote/src/main/scala/akka/remote/RemoteDaemon.scala

@@ -135,8 +135,6 @@ private[akka] class RemoteSystemDaemon(
 
     case Identify(messageId) ⇒ sender ! ActorIdentity(messageId, Some(this))
 
-    case t: Terminated       ⇒
-
     case TerminationHook ⇒
       terminating.switchOn {
         terminationHookDoneWhenNoChildren()