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
* Welcome message instead of gossip as reply to join, only accept Welcome from the member that we are trying to join with
* Create uid in RARP
* Use uid in cluster Member identifier, UniqueAddress
* Use uid in vclock identifier
* Ignore gossips to wrong destination (uid), and from unknown (and unreachable) members
* Make sure received gossip contains selfAddress
* Test join of fresh node with same host:port
* Remove JoinTwoClustersSpec
* Retry unsucessful join request
* Make Member apply and constructor is private[akka]