+act - 15757 - Reworks implementation of ActorSystem shutdown

* deprecates awaitTermination, shutdown and isTerminated
  * introduces a terminate-method that returns a Future[Unit]
  * introduces a whenTerminated-method that returns a Future[Unit]
  * simplifies the implementation by removing blocking constructs
  * adds tests for terminate() and whenTerminated

akka-samples/akka-sample-main-scala/src/main/scala/sample/hello/Main2.scala

@@ -20,7 +20,7 @@ object Main2 {
     def receive = {
       case Terminated(_) =>
         log.info("{} has terminated, shutting down system", ref.path)
-        context.system.shutdown()
+        context.system.terminate()
     }
   }