Disallow re-joining, see #2873

Doesn't cover all scenarios yet, but is a great improvement in itself. I will discuss one scenario at akka-dev.

* 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