Race in creating new remote child, see #3423

* RemoteSystemDaemon removes its child when sendSystemMessage
  DeathWatchNotification is called from tellWatchersWeDied.
  That can happen after Terminated is delivered to other watcher,
  which create a new child with same name, and then the new child
  can then be removed instead of the old one.
* RemoteSystemDaemon must only remove child with exact match of
  full ActorRef
Merge pull request #1519 from akka/wip-3423-child-race-patriknw

Race in creating new remote child, see #3423