+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/scala/code/docs/persistence/PersistenceDocSpec.scala

@@ -322,7 +322,7 @@ trait PersistenceDocSpec {
 
     processor ! PersistentBatch(List(Persistent("a"), Persistent("b")))
     //#batch-write
-    system.shutdown()
+    system.terminate()
   }
 
   new AnyRef {