+con #3937 Start ShardCoordinator again after PersistenceFailure

* Add supervisor level that will start the ShardCoordinator again after
  a configurable backoff duration
* Make the timeout of SharedLeveldbJournal configurable
* Include cause of PersistenceFailure in message of ActorKilledException

(cherry picked from commit 8ebc413643c5b1e755057ecab7e660f027bfd232)
Merge pull request #2105 from akka/wip-3937-sharding-persistent-failure-2.3-patriknw

+con #3937 Start ShardCoordinator again after PersistenceFailure (backport)