New Snapshot Store Schema

[octonato]

Like the journal store, the new snapshot will have a few more fields:

• ser_id (Int)
• ser_manifest (String)
• snapshot_payload (String)

We need to refactor the Snapshot DAO for that new format.

The current table is snapshot. Because we don't want destructive migrations, we can add a new schema table called snapshots. New users can just start using it from version 4.0.0 onward. Existing users will need to run the migration procedure to copy their data from snapshot to snapshots.

ByteArraySnapshotSerializer should be moved to the migration module as we will need it for running migrations (reading old data from snapshot table).

[chbatey]

#467