Stores a ClientStats instance.

def WriteClientStats(self,
                       client_id,
                       stats,
                       cursor=None):
    """Stores a ClientStats instance."""

    try:
      cursor.execute(
          """
          INSERT INTO client_stats (client_id, payload, timestamp)
          VALUES (%s, %s, FROM_UNIXTIME(%s))
          ON DUPLICATE KEY UPDATE payload=VALUES(payload)
          """, [
              db_utils.ClientIDToInt(client_id),
              stats.SerializeToString(),
              mysql_utils.RDFDatetimeToTimestamp(rdfvalue.RDFDatetime.Now())
          ])
    except MySQLdb.IntegrityError as e:
      if e.args[0] == mysql_error_constants.NO_REFERENCED_ROW_2:
        raise db.UnknownClientError(client_id, cause=e)
      else:
        raise