Remove SelfActorRef and use ActorContext to access state in ActorInstance. See #1202

- removed SelfActorRef
- added an ActorContext interface that provides a view into ActorInstance
- the ActorInstance (as ActorContext) gets injected into the Actor
- moved self methods like sender and reply directly to the Actor

akka-tutorials/akka-tutorial-first/src/main/scala/Pi.scala

@@ -40,7 +40,7 @@ object Pi extends App {
 
     def receive = {
       case Work(start, nrOfElements) ⇒
-        self reply Result(calculatePiFor(start, nrOfElements)) // perform the work
+        reply(Result(calculatePiFor(start, nrOfElements))) // perform the work
     }
   }