+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/testkit/TestkitDocSpec.scala

@@ -284,7 +284,7 @@ class TestkitDocSpec extends AkkaSpec with DefaultTimeout with ImplicitSender {
       //#put-your-test-code-here
       val probe = TestProbe()
       probe.send(testActor, "hello")
-      try expectMsg("hello") catch { case NonFatal(e) => system.shutdown(); throw e }
+      try expectMsg("hello") catch { case NonFatal(e) => system.terminate(); throw e }
       //#put-your-test-code-here
 
       shutdown(system)