Deletes up to `limit` ClientStats older than `retention_time`.

def _DeleteClientStats(self,
                         limit,
                         retention_time,
                         cursor=None):
    """Deletes up to `limit` ClientStats older than `retention_time`."""
    cursor.execute(
        "DELETE FROM client_stats WHERE timestamp < FROM_UNIXTIME(%s) LIMIT %s",
        [mysql_utils.RDFDatetimeToTimestamp(retention_time), limit])
    return cursor.rowcount