+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
Merge pull request #2097 from akka/wip-3937-sharding-persistent-failure-patriknw

+con #3937 Start ShardCoordinator again after PersistenceFailure