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-samples/akka-sample-cluster/src/multi-jvm/scala/sample/cluster/stats/StatsSampleSpec.scala

@@ -97,10 +97,8 @@ abstract class StatsSampleSpec extends MultiNodeSpec(StatsSampleSpecConfig)
       system.actorOf(Props[StatsWorker], "statsWorker")
       system.actorOf(Props[StatsService], "statsService")
 
-      expectMsgAllOf(
-        MemberUp(Member(firstAddress, MemberStatus.Up, Set.empty)),
-        MemberUp(Member(secondAddress, MemberStatus.Up, Set.empty)),
-        MemberUp(Member(thirdAddress, MemberStatus.Up, Set.empty)))
+      receiveN(3).collect { case MemberUp(m) => m.address }.toSet must be (
+          Set(firstAddress, secondAddress, thirdAddress))
 
       Cluster(system).unsubscribe(testActor)