fix bug of settimeout

[MBCmbc]

   In the original code, new timeout value is equal to "self._rtt_a * 2". If the last "self._rtt_a" is very small(such as 50 ms), then the new timeout will be too small, thus result in misjudgment of timeout. 
   I think there should be a minimum threshold(such as 3 seconds) of timeout to avoid misjudgment.