=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)
=con #15285 wrap message in RouterEnvelope before routing

PubSubMediator uses router which always unwraps RouterEnvelope messages.
However unwrapping is undesirable if user sends message in
ConsistentHashableEnvelope. Thus PubSubMediator should always wrap user
messages in RouterEnvelope which will be unwrapped by the router, leaving
user message unchanged.

Also disallow consistent hashing routing logic in pub-sub mediator.
=con Use separate probes in ReliableProxyDocSpec
Merge pull request #15352 from akka/wip-ReliableProxyDocSpec-patriknw

=con Use separate probes in ReliableProxyDocSpec
Merge pull request #15300 from 2m/pubsub-wrap-before-routing-23

=con #15285 wrap message in RouterEnvelope before routing
+act #13992 Improve identify replies of wildcard selections