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-docs/rst/cluster/cluster-usage-java.rst

@@ -126,6 +126,17 @@ seed nodes at all.
 
 Joining can also be performed programatically with ``Cluster.get(system).join(address)``.
 
+Unsuccessful join attempts are automatically retried after the time period defined in 
+configuration property ``retry-unsuccessful-join-after``. When using auto-joining with
+``seed-nodes`` this means that a new seed node is picked. When joining manually or
+programatically this means that the last join request is retried. Retries can be disabled by
+setting the property to ``off``.
+
+An actor system can only join a cluster once. Additional attempts will be ignored.
+When it has successfully joined it must be restarted to be able to join another
+cluster or to join the same cluster again. It can use the same host name and port
+after the restart, but it must have been removed from the cluster before the join
+request is accepted.
 
 Automatic vs. Manual Downing
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^