=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
one question to @rkuhn: `boss` is a systemActor, what does mean to attach a child systemService like this? When I create the child with the MkChild message I use ordinary actorOf. Does this make any difference?