With the pacing behaviour of the ICE RFC 5245, the stun packets are
send with an initial timeout that cannot be lower that 100ms, and that
increases of 20ms for each new in-progress/waiting conncheck above 5.
Typical initial timeout is in the range 100ms - 400ms (obtained when
there are 20 in-progress/waiting connchecks).

The rationale with this modification is that we consider that 4
retransmissions with an initial timeout of 100ms and a last timeout of 4
* 100 ms, gives (1 + 2 + 4 + 8 + 4) * 100ms = 1900ms for a response to
be received after the initial stun request has been sent, which is
probably enough in most situations. With an initial timeout of 400ms,
this delay extends to 7600ms (this is proportional to the value of the
initial timeout).

Differential Revision: https://phabricator.freedesktop.org/D1109