Clean the syncv3_snapshots table periodically

[kegsay]

Also cleans the transaction table periodically.

Fixes #372

On testing, this cuts db size to about 1/3 of its original size.

[melroy89]

This query:

WITH ranked_snapshots AS (
		SELECT
		  snapshot_id,
		  room_id,
		  ROW_NUMBER() OVER (PARTITION BY room_id ORDER BY snapshot_id DESC) AS row_num
		FROM
		  syncv3_snapshots
	  )
	  DELETE FROM syncv3_snapshots USING ranked_snapshots
	  WHERE syncv3_snapshots.snapshot_id = ranked_snapshots.snapshot_id
	  AND ranked_snapshots.row_num > 51;

Is running REALLY REALLY long:

(discovered via: SELECT pid, now() - pg_stat_activity.query_start AS duration, query 
FROM pg_stat_activity 
WHERE (now() - pg_stat_activity.query_start) > interval '1 minute';
)

@kegsay Is this normal!?? Are you missing an index maybe on table?