Implements neighbor configuration change listener.

def on_update_enabled(self, conf_evt):
        """Implements neighbor configuration change listener.
        """
        enabled = conf_evt.value
        # If we do not have any protocol bound and configuration asks us to
        # enable this peer, we try to establish connection again.
        if enabled:
            LOG.info('%s enabled', self)
            if self._protocol and self._protocol.started:
                LOG.error('Tried to enable neighbor that is already enabled')
            else:
                self.state.bgp_state = const.BGP_FSM_CONNECT
                # Restart connect loop if not already running.
                if not self._connect_retry_event.is_set():
                    self._connect_retry_event.set()
                    LOG.debug('Starting connect loop as neighbor is enabled.')
        else:
            LOG.info('%s disabled', self)
            if self._protocol:
                # Stopping protocol will eventually trigger connection_lost
                # handler which will do some clean-up.
                # But the greenlet that is in charge of the socket may be kill
                # when we stop the protocol, hence we call connection_lost
                # here as we triggered socket to close.
                self._protocol.send_notification(
                    BGP_ERROR_CEASE,
                    BGP_ERROR_SUB_ADMINISTRATIVE_SHUTDOWN
                )
                self._protocol.stop()
                self._protocol = None
                self.state.bgp_state = const.BGP_FSM_IDLE
            # If this peer is not enabled any-more we stop trying to make any
            # connection.
            LOG.debug('Disabling connect-retry as neighbor was disabled')
            self._connect_retry_event.clear()