Cluster singleton failure due to down-removed, see #3130

* The scenario was that previous leader left.
* The problem was that the new leader got MemberRemoved
  before it got the HandOverDone and therefore missed the
  hand over data.
* Solved by not changing the singleton to leader when receiving
  MemberRemoved and instead do that on normal HandOverDone or
  in failure cases after retry timeout.
* The reason for this bug was the new transition from Down to
  Removed and that there is now no MemberDowned event. Previously
  this was only triggered by MemberDowned (not MemberRemoved) and
  that was safe because that was "always" preceeded by unreachable.
* The new solution means that it will take longer for new singleton
  to startup in case of unreachable previous leader, but I don't
  want to trigger it on MemberUnreachable because it might in the
  future be possible to switch it back to reachable.
Merge pull request #1235 from akka/wip-3130-cluster-singleton-rm-prev-leader-patriknw

Cluster singleton failure due to down-removed, see #3130