!per #3717 Deletion API for snapshots and persistent messages

- single and bulk deletion of messages
- single and bulk deletion of snapshots
- run journal and snapshot store as system actors
- rename physical parameter in delete methods to permanent
- StashSupport.prepend docs and implementation enhancements
!per #3729 Journaling protocol optimization

- internal batching of individually received Persistent messages
- testing fault tolerance of Processor in presence of random
  * journaling failures
  * processing failures