=act #15409 Fix race in ActorDSL

* The reason it didn't work is that messages are delivered before the cell is swapped
  as can be seen in RepointableActorRef.replaceWith (L195)
* Use ask until the RepointableActorRef is started, then use attachChild
systemService only prevents deployment configuration from being applied, otherwise `attachChild` is equivalent to `context.actorOf` apart from the timing