Audit log: retention policy + old entries cleanup

[david-crespo]

Background task to delete entries older than N days, as determined by our as-yet-undetermined our retention policy. We need to keep an eye on how fast the table will grow, but it seems we already have some tables that are quite huge compared to this one and we don't clean them up yet, so this is not a very high priority. We expect customers will want to frequently fetch the log and save it off-rack, so the retention period probably doesn't need to be very long.