stun: set delay in retransmission instead of adding it

We may have situation when stun_timer_refresh is called with a significant delay after the current deadline. In the actual behaviour, this delay is just included to the computation of the new deadline of the next stun retransmission. We think this may lead to unfair situations, where the next deadline may be too short, just to compensate the first deadline that was too long.

For example, if a stun request is scheduled with a delay of 200ms for the 2nd transmission, and 400ms for the 3rd transmission, if stun_timer_remainder() is called 300ms after the start of the timer, the second delay will last only 300ms, instead of 400ms.