!per #3631 Snapshotting

- capture and save snapshots of processor state
- start processor recovery from saved snapshots
- snapshot storage on local filesystem
- snapshot store completely isolated from journal
- LevelDB journal modularized (and completely re-rwritten)
- In-memory journal removed
Merge pull request #1747 from eligosource/wip-3631-snapshotting-krasserm

!per #3631 Snapshotting