conncheck: update stun timer timeout for RFC8445

conncheck: update stun timer timeout for RFC8445

The new version of the RFC suppressed the difference between reliable
and not reliable maximum value for RTO. We choose to keep the value of
100ms that we used previously, which is lower that the recommended
value, but could be overriden most of the time, when a significant
number of pairs are handled.

We also compute exactly the number of in-progress and waiting
pairs for all streams of the agent, without relying on the value
per-stream, multiplied by the number of active streams.