=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters

(cherry picked from commit b2f96668baf9efa77de5f97223f055c0a78d0cb8)
Merge pull request #15348 from akka/wip-13678-identify-deadLetters-master-patriknw

=act #13678 #15149 Reply with ActorIdentity(None) from deadLetters (for verification)
=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)
Merge pull request #15376 from akka/wip-13898-failed-SharedLeveldbJournalSpec-master-ban

=per #13898 Make LocalSnapshotStore directory creation more robust (Validation)
=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.