!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
Merge pull request #1838 from krasserm/wip-3717-deletion-api-krasserm

!per #3717 Deletion API for snapshots and persistent messages