make system.actorOf() non-blocking (and working), see #2031

- introducing RepointableActorRef, which starts out with an
  UnstartedActorCell which can cheaply be created; the Supervise()
  message will trigger child.activate() in the supervisor, which means
  that the actual creation (now with normal ActorCell) happens exactly
  in the right place and with the right semantics. Messages which were
  enqueued to the dummy cell are transferred atomically into the
  ActorCell (using normal .tell()), so message sends keep working
  exactly as they used to
- this enables getting rid of the brittle synchronization around
  RoutedActorRef by replacing that one with a RepointableActorRef
  subclass which creates RoutedActorCells upon activate(), with the nice
  benefit that there is no hurry then to get it right because the new
  cell is constructed “on the side”

misc fixes:

- InvalidMessageException is now actually enforced when trying to send
  “null”
- Mailboxes may be created without having an ActorCell, which can come
  in handy later, because the cell is only needed when this mailbox is
  going to be scheduled on some executor
- remove occurrences of Props(), which is equivalent to Props[Nothing],
  which is equivalent to «bug»
- add test case which verifies that context.actorOf is still synchronous
- plus all the stuff I have forgotten.

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

@@ -16,8 +16,10 @@ import akka.event.Logging.LogEventException
 import akka.jsr166y.{ ForkJoinTask, ForkJoinPool }
 import akka.util.{ Unsafe, Duration, NonFatal, Index }
 
-final case class Envelope(val message: Any, val sender: ActorRef)(system: ActorSystem) {
-  if (message.isInstanceOf[AnyRef]) {
+final case class Envelope private (val message: Any, val sender: ActorRef)
+
+object Envelope {
+  def apply(message: Any, sender: ActorRef, system: ActorSystem): Envelope = {
     val msg = message.asInstanceOf[AnyRef]
     if (msg eq null) throw new InvalidMessageException("Message is null")
     if (system.settings.SerializeAllMessages && !msg.isInstanceOf[NoSerializationVerificationNeeded]) {
@@ -30,6 +32,7 @@ final case class Envelope(val message: Any, val sender: ActorRef)(system: ActorS
         }
       }
     }
+    new Envelope(message, sender)
   }
 }
 
@@ -228,8 +231,8 @@ private[akka] object MessageDispatcher {
     } {
       val status = if (a.isTerminated) " (terminated)" else " (alive)"
       val messages = a match {
-        case l: LocalActorRef ⇒ " " + l.underlying.mailbox.numberOfMessages + " messages"
-        case _                ⇒ " " + a.getClass
+        case r: ActorRefWithCell ⇒ " " + r.underlying.numberOfMessages + " messages"
+        case _                   ⇒ " " + a.getClass
       }
       val parent = a match {
         case i: InternalActorRef ⇒ ", parent: " + i.getParent
@@ -265,7 +268,7 @@ abstract class MessageDispatcher(val prerequisites: DispatcherPrerequisites) ext
   /**
    *  Creates and returns a mailbox for the given actor.
    */
-  protected[akka] def createMailbox(actor: ActorCell): Mailbox //FIXME should this really be private[akka]?
+  protected[akka] def createMailbox(actor: Cell): Mailbox //FIXME should this really be private[akka]?
 
   /**
    * Identifier of this dispatcher, corresponds to the full key