=rem #13923 Fixes hanging ActorSystem termination

* occurs on failed RemoteActorRef initialization
* fixed shutting down Netty when startup fails
* fixed registering ActorSystem's termination hook before ActorRefProvider
  initialization
* added new test case (checking if no threads left after a failed actor
  system initialization)

(cherry picked from commit 5e978220a5b0bd5c49668bb4b4db3808f27c6ce2)

Conflicts:
	akka-actor/src/main/scala/akka/actor/ActorSystem.scala
Merge pull request #15344 from akka/wip-13923-backport-patriknw

=rem #13923 Fixes hanging ActorSystem termination (backport)