=con #3865 Fix race in pub-sub when nodes are removed

* The race can happen if the MemberRemoved event is received followed by a Delta update from
  a node that has not yet got the MemberRemoved. That will make the bucket for the removed
  node to be added back in the registry.
Merge pull request #1995 from akka/wip-3865-remove-race-pubsub-patriknw

=con #3865 Fix race in pub-sub when nodes are removed