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-docs/scala/code/docs/dispatcher/DispatcherDocSpec.scala

@@ -135,7 +135,7 @@ object DispatcherDocSpec {
 
   //#mailbox-implementation-example
   case class MyUnboundedMailbox() extends akka.dispatch.MailboxType {
-    import akka.actor.ActorContext
+    import akka.actor.{ ActorRef, ActorSystem }
     import com.typesafe.config.Config
     import java.util.concurrent.ConcurrentLinkedQueue
     import akka.dispatch.{
@@ -149,7 +149,7 @@ object DispatcherDocSpec {
     def this(settings: ActorSystem.Settings, config: Config) = this()
 
     // The create method is called to create the MessageQueue
-    final override def create(owner: Option[ActorContext]): MessageQueue =
+    final override def create(owner: Option[ActorRef], system: Option[ActorSystem]): MessageQueue =
       new QueueBasedMessageQueue with UnboundedMessageQueueSemantics {
         final val queue = new ConcurrentLinkedQueue[Envelope]()
       }