Hold timer expired event handler.

def _expired(self):
        """Hold timer expired event handler.
        """
        LOG.info('Negotiated hold time %s expired.', self._holdtime)
        code = BGP_ERROR_HOLD_TIMER_EXPIRED
        subcode = BGP_ERROR_SUB_HOLD_TIMER_EXPIRED
        self.send_notification(code, subcode)
        self.connection_lost('Negotiated hold time %s expired.' %
                             self._holdtime)
        self.stop()