Publish LeaderChanged when first seen, see #3131

* The problem in ClusterSingletonManagerChaosSpec was that node 4 doesn't publish
  LeaderChanged, because there is never convergence on node 4 of the new Up
  state for the three new nodes before they are shutdown. When it becomes
  convergence on node 4 prevConvergedGossip and newGossip have same leader
  (i.e. no change).
* LeaderChanged is now published when the new leader is first seen, i.e. same
  as member events. This makes sense now when leader can't be in Joining state.
Merge pull request #1237 from akka/wip-3131-pub-leaderchanged-patriknw

Publish LeaderChanged when first seen, see #3131