improve DeadLetter reporting

(since I know now what’s causing these Jenkins failures ;-) )

- include recipient in DeadLetter
- include recipient in calls to enqueue/systemEnqueue
- move DeadLetterMailbox to ActorSystem (saves some space, too)
- hook up DeadLetterMailbox so it sends DeadLetters to app.deadLetters,
  which publishes them on the eventStream
- subscribe TestEventListener to DeadLetter and turn it into Warning

The generated warnings about ChildTerminated are very much correct, they
remind us that we still need to fix supervisor.stop() to await all
children’s death before actually committing suicide.

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

@@ -100,21 +100,9 @@ abstract class MessageDispatcher(val app: ActorSystem) extends Serializable {
   protected[akka] def createMailbox(actor: ActorCell): Mailbox
 
   /**
-   * Create a blackhole mailbox for the purpose of replacing the real one upon actor termination
+   * a blackhole mailbox for the purpose of replacing the real one upon actor termination
    */
-  protected[akka] val deadLetterMailbox: Mailbox = DeadLetterMailbox
-
-  object DeadLetterMailbox extends Mailbox(null) {
-    becomeClosed()
-    override def dispatcher = null //MessageDispatcher.this
-    override def enqueue(envelope: Envelope) = ()
-    override def dequeue() = null
-    override def systemEnqueue(handle: SystemMessage): Unit = ()
-    override def systemDrain(): SystemMessage = null
-    override def hasMessages = false
-    override def hasSystemMessages = false
-    override def numberOfMessages = 0
-  }
+  import app.deadLetterMailbox
 
   /**
    * Name of this dispatcher.
@@ -225,7 +213,7 @@ abstract class MessageDispatcher(val app: ActorSystem) extends Serializable {
         // message must be “virgin” before being able to systemEnqueue again
         val next = message.next
         message.next = null
-        deadLetterMailbox.systemEnqueue(message)
+        deadLetterMailbox.systemEnqueue(actor.self, message)
         message = next
       }
     }
@@ -233,7 +221,7 @@ abstract class MessageDispatcher(val app: ActorSystem) extends Serializable {
     if (mailBox.hasMessages) {
       var envelope = mailBox.dequeue
       while (envelope ne null) {
-        deadLetterMailbox.enqueue(envelope)
+        deadLetterMailbox.enqueue(actor.self, envelope)
         envelope = mailBox.dequeue
       }
     }