Only quarantine removed member that was unreachable, see #2594

* For graceful leaving and remove it should still be possible to
  communicate with the node after cluster removal.
* Otherwise the hand over in cluster singleton would break, for
  example.
* Also, skip selfAddress to avoid generation of AddressTerminated
  for the own node when removed from cluster.

It might be useful with a flag in MemberRemoved to tell if it was the unreachable/down path or the graceful leaving path. I will create a separate ticket for that, which can be considered later.