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/modules/code/docs/actor/mailbox/DurableMailboxDocSpec.scala

@@ -11,7 +11,7 @@ import akka.actor.Props
 import org.scalatest.{ BeforeAndAfterAll, WordSpec }
 import org.scalatest.matchers.MustMatchers
 import akka.testkit.AkkaSpec
-import akka.actor.Actor
+import akka.actor.{ Actor, ExtendedActorSystem }
 
 class MyActor extends Actor {
   def receive = {
@@ -56,20 +56,20 @@ import akka.util.duration._
 class MyMailboxType(systemSettings: ActorSystem.Settings, config: Config)
   extends MailboxType {
 
-  override def create(owner: Option[ActorContext]): MessageQueue = owner match {
-    case Some(o) ⇒ new MyMessageQueue(o)
+  override def create(owner: Option[ActorRef], system: Option[ActorSystem]): MessageQueue = owner zip system headOption match {
+    case Some((o, s: ExtendedActorSystem)) ⇒ new MyMessageQueue(o, s)
     case None ⇒ throw new IllegalArgumentException(
       "requires an owner (i.e. does not work with BalancingDispatcher)")
   }
 }
 
-class MyMessageQueue(_owner: ActorContext)
-  extends DurableMessageQueue(_owner) with DurableMessageSerialization {
+class MyMessageQueue(_owner: ActorRef, _system: ExtendedActorSystem)
+  extends DurableMessageQueue(_owner, _system) with DurableMessageSerialization {
 
   val storage = new QueueStorage
   // A real-world implmentation would use configuration to set the last 
   // three parameters below
-  val breaker = CircuitBreaker(_owner.system.scheduler, 5, 30.seconds, 1.minute)
+  val breaker = CircuitBreaker(system.scheduler, 5, 30.seconds, 1.minute)
 
   def enqueue(receiver: ActorRef, envelope: Envelope): Unit = breaker.withSyncCircuitBreaker {
     val data: Array[Byte] = serialize(envelope)
@@ -91,7 +91,7 @@ class MyMessageQueue(_owner: ActorContext)
    * but the purpose of a durable mailbox is to continue
    * with the same message queue when the actor is started again.
    */
-  def cleanUp(owner: ActorContext, deadLetters: MessageQueue): Unit = ()
+  def cleanUp(owner: ActorRef, deadLetters: MessageQueue): Unit = ()
 
 }
 //#custom-mailbox