=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
Merge pull request #15754 from akka/wip-15409-ActorDSL-race-patriknw

=act #15409 Fix race in ActorDSL