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-cluster/src/main/scala/akka/cluster/ClusterReadView.scala

@@ -70,8 +70,9 @@ private[akka] class ClusterReadView(cluster: Cluster) extends Closeable {
   }
 
   def self: Member = {
-    state.members.find(_.address == selfAddress).orElse(state.unreachable.find(_.address == selfAddress)).
-      getOrElse(Member(selfAddress, MemberStatus.Removed, cluster.selfRoles))
+    import cluster.selfUniqueAddress
+    state.members.find(_.uniqueAddress == selfUniqueAddress).orElse(state.unreachable.find(_.uniqueAddress == selfUniqueAddress)).
+      getOrElse(Member(selfUniqueAddress, cluster.selfRoles).copy(status = MemberStatus.Removed))
   }
 
   /**