+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/io/EchoServer.scala

@@ -22,7 +22,7 @@ object EchoServer extends App {
 
   // make sure to stop the system so that the application stops
   try run()
-  finally system.shutdown()
+  finally system.terminate()
 
   def run(): Unit = {
     import ActorDSL._