+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/actor/japi/FaultHandlingDocSample.java

@@ -22,7 +22,6 @@ import com.typesafe.config.ConfigFactory;
 
 import static akka.japi.Util.classTag;
 
-import static akka.actor.SupervisorStrategy.resume;
 import static akka.actor.SupervisorStrategy.restart;
 import static akka.actor.SupervisorStrategy.stop;
 import static akka.actor.SupervisorStrategy.escalate;
@@ -76,12 +75,12 @@ public class FaultHandlingDocSample {
         log.info("Current progress: {} %", progress.percent);
         if (progress.percent >= 100.0) {
           log.info("That's all, shutting down");
-          getContext().system().shutdown();
+          getContext().system().terminate();
         }
       } else if (msg == ReceiveTimeout.getInstance()) {
         // No progress within 15 seconds, ServiceUnavailable
         log.error("Shutting down due to unavailable service");
-        getContext().system().shutdown();
+        getContext().system().terminate();
       } else {
         unhandled(msg);
       }

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();
     }
   }