Exiting members must still have seen the gossip for convergence, see #3171

* Otherwise MemberExited might not be published on the leaving node,
  since the leader may act Leaving->Exiting->Removed without
  the leaving node seeing and publishing the transition.
Merge pull request #1274 from akka/wip-3171-convergence-exiting-patriknw

Exiting members must still have seen the gossip for convergence, see #3171