remove ActorRef from Failed/ChildTerminated and make some warnings nicer

- the same information is transmitted as sender, hence enabling
  ChildTerminated to become a singleton
- make lastSender accessible in TestKit (needed now for DeathWatchSpec)
- fix nasty infinite loop when logging at the wrong moment during
  shutdown

akka-actor/src/main/scala/akka/dispatch/AbstractDispatcher.scala

@@ -196,8 +196,8 @@ abstract class MessageDispatcher(val app: ActorSystem) extends Serializable {
   protected[akka] def unregister(actor: ActorCell) {
     _actors.decrementAndGet()
     val mailBox = actor.mailbox
-    mailBox.becomeClosed()
-    actor.mailbox = deadLetterMailbox //FIXME getAndSet would be preferrable here
+    actor.mailbox = deadLetterMailbox
+    mailBox.becomeClosed() // FIXME reschedule in tell if possible race with cleanUp is detected in order to properly clean up 
     cleanUpMailboxFor(actor, mailBox)
   }