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-testkit/src/test/scala/akka/testkit/AkkaSpec.scala

@@ -76,7 +76,7 @@ abstract class AkkaSpec(_system: ActorSystem)
   protected def atTermination() {}
 
   def spawn(body: ⇒ Unit)(implicit dispatcher: MessageDispatcher) {
-    system.actorOf(Props(ctx ⇒ { case "go" ⇒ try body finally ctx.self.stop() }).withDispatcher(dispatcher)) ! "go"
+    system.actorOf(Props(ctx ⇒ { case "go" ⇒ try body finally ctx.stop(ctx.self) }).withDispatcher(dispatcher)) ! "go"
   }
 }
 
@@ -120,7 +120,7 @@ class AkkaSpecSpec extends WordSpec with MustMatchers {
         implicit val davyJones = otherSystem.actorOf(Props(new Actor {
           def receive = {
             case m: DeadLetter ⇒ locker :+= m
-            case "Die!"        ⇒ sender ! "finally gone"; self.stop()
+            case "Die!"        ⇒ sender ! "finally gone"; context.stop(self)
           }
         }), "davyJones")