Transition from Down to Removed, see #3075
Publish member events when state change first seen, see #3075

* Remove InstantMemberEvent
Joining member should not be leader, see #3021

* Prefer members with status Up or Leaving, and as
  fallback use Joining, Exiting, Down
* Minor scaladoc fixes
Merge pull request #1224 from akka/wip-3075-pub-member-events-patriknw

Publish member events when state change first seen, see #3075