+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
+act - 15757 - Changes the signature of ActorSystem.terminate() and ActorSystem.whenTerminated() to be of Future[Terminated] iso Future[Unit]
Merge pull request #15758 from akka/wip-15757-refactor-actorsystem-termination-√

Wip 15757 refactor actorsystem termination √