+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-docs/rst/java/code/docs/io/japi/EchoServer.java

@@ -28,7 +28,7 @@ public class EchoServer {
       watcher.tell(ackServer, ActorRef.noSender());
       latch.await(10, TimeUnit.MINUTES);
     } finally {
-      system.shutdown();
+      system.terminate();
     }
   }