+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._

akka-docs/rst/scala/code/docs/io/ReadBackPressure.scala

@@ -10,6 +10,9 @@ import java.net.InetSocketAddress
 import akka.testkit.{ ImplicitSender, TestProbe, AkkaSpec }
 import akka.util.ByteString
 
+import scala.concurrent.Await
+import scala.concurrent.duration.Duration
+
 object PullReadingExample {
 
   class Listener(monitor: ActorRef) extends Actor {
@@ -77,7 +80,6 @@ class PullReadingSpec extends AkkaSpec with ImplicitSender {
     client.send(connection, ResumeReading)
     client.expectMsg(Received(ByteString("hello")))
 
-    system.shutdown()
-    system.awaitTermination
+    Await.ready(system.terminate(), Duration.Inf)
   }
 }