=rem #3726 Fix race in RemoteWatcher for first quick watch/unwatch

* The problem scenario was that a remote watch followed by re-watch triggered by
  first heartbeat and unwatch coming in before the extra re-watch message. That
  caused RemoteWatcher to still watch the subject even though it was intended to
  be unwatched.
* I could reproduce it with sleeps at stratgic points
* Sovled by separate re-watch message and check that still watching
Merge pull request #1866 from akka/wip-3726-remote-watch-cleanup-failure-patriknw

=rem #3726 Fix race in RemoteWatcher for first quick watch/unwatch