=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)
=per #15212 Avoid half written snapshots in LocalSnapshotStore

Conflicts:
	project/AkkaBuild.scala
=per #15131 Make LocalSnapshotStore fail if it can't create target directory
Merge pull request #15323 from akka/wip-15212-make-LocalSnapshotStore-more-robust-master-ban

=per #15212 Avoid half written snapshots in LocalSnapshotStore (Forward Port)
Merge pull request #15324 from akka/wip-15131-snapshotstore-directory-check-master-ban

=per #15131 Make LocalSnapshotStore fail if it can't create target directory (Forward Port)
=proj #15333 Update to sbt 0.13.5
Merge pull request #15334 from 2m/wip-update-to-sbt-0.13.5

=proj #15333 Update to sbt 0.13.5
Merge pull request #15112 from csomogyi/master

=rem #13923 Fixes hanging ActorSystem termination