Multiple snapshots

In single Snapshot mode, the Snapshot is the current state of Monitor DB. In this mode, at every checkpoint, the Snapshot must be emptied.

In multiple Snapshots mode, a snapshot is a DB. Name pattern is the following:
<monitor_name>_<thread_id>_<snapshot_id>

If multi-worker mode is on, <thread_id> is CONNECTION_ID(); else, it is an empty string.

If multi-snapshot mode is on, <snapshot_id> is a progressive number.

Add options:
* multiworker: boolean
* multisnapshot: boolean

Add procedure:
void purge(): drop all snapshots created by this worker.