=clu #3603 Handle removed member in Gossip and Reachability merge

* It was a regression introduced in dc9fe4f
* Two problems:
  1) Gossip merge could pop back removed member (was previously
     covered by the filter of unreachable)
  2) Reachability merge didn't handle all cases for removed member,
     i.e. when node not in allowed set
Merge pull request #1731 from akka/wip-3603-ClientDowningNodeThatIsUnreachableSpec-patriknw

=clu #3603 Handle removed member in Gossip and Reachability merge