remove ActorRef.stop()

- replace ActorRef.stop() by ActorRefFactory.stop(child) everywhere
- ActorCell “optimizes” this to remove the child from its childrenRefs
  in order to allow immediate recycling of the name
- the lost soul must have a place, for which the Locker has been
  created, where Davy Jones will happily rebind the soul to his ship
  (i.e. set “parent” to the locker to avoid mem leak) and periodically
  revisit it (.stop(), in case of that being lost in comm failure,
  similar .watch() to re-check liveness)

akka-docs/common/code/SchedulerDocSpec.scala

@@ -62,6 +62,6 @@ class SchedulerDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
     //This cancels further Ticks to be sent
     cancellable.cancel()
     //#schedule-recurring
-    tickActor.stop()
+    system.stop(tickActor)
   }
 }