=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
=pro Add watchSources for *.rst to akka-docs
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
=rem #13923 Mute log and stack traces in RemoteInitErrorSpec
Merge pull request #15345 from akka/wip-13923-silence-test-log-patriknw

=rem #13923 Mute log and stack traces in RemoteInitErrorSpec
=tes #15357 fix for PrettyDurationSpec
=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters

(cherry picked from commit b2f96668baf9efa77de5f97223f055c0a78d0cb8)
Merge pull request #15335 from akka/wip-add-watchSources-for-rst-ban

=pro Add watchSources for *.rst to akka-docs (Forward Port)
Merge pull request #15358 from mkubala/PrettyDurationSpec-fix-mkubala

=tes #15357 fix for PrettyDurationSpec
Merge pull request #15348 from akka/wip-13678-identify-deadLetters-master-patriknw

=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters (for verification)
+tes #15132 Add additional overloaded expectMsg to TestKit
=per #13962 Close LevelDB snapshots to avoid resource leakage
=per #13898 Make LocalSnapshotStore directory creation more robust
Merge pull request #15375 from akka/wip-13962-investigate-leveldb-compaction-issue-master-ban

=per #13962 Close LevelDB snapshots to avoid resource leakage (Validation)