Update transmission period of the BFD session.

def _update_xmit_period(self):
        """
        Update transmission period of the BFD session.
        """
        # RFC5880 Section 6.8.7.
        if self._desired_min_tx_interval > self._remote_min_rx_interval:
            xmit_period = self._desired_min_tx_interval
        else:
            xmit_period = self._remote_min_rx_interval

        # This updates the transmission period of BFD Control packets.
        # (RFC5880 Section 6.8.2 & 6.8.3.)
        if self._detect_mult == 1:
            xmit_period *= random.randint(75, 90) / 100.0
        else:
            xmit_period *= random.randint(75, 100) / 100.0

        self._xmit_period = xmit_period / 1000000.0
        LOG.info("[BFD][%s][XMIT] Transmission period changed to %f",
                 hex(self._local_discr), self._xmit_period)