stun: update timer timeout and retransmissions

stun: update timer timeout and retransmissions

This patch updates the stun timing constants and provides the rationale
with the choice of these new values, in the context of the ice
connection check algorithm.

One important value during the discovery state is the combination of the
initial timeout and the number of retransmissions, because this state
may complete after the last stun discovery binding request has timed
out. With the combination of 500ms and 3 retransmissions, the discovery
state is bound to 2000ms to discover server reflexive and relay
candidates.

The retransmission delay doubles at each retransmission except for the
last one. Generally, this state will complete sooner, when all
discovery requests get a reply before the timeout.

Another mechanism is used during the connection check, where an stun
request is sent with an initial timeout defined by :

   RTO = MAX(500ms, Ta * (number of in-progress + waiting pairs))
   with Ta = 20ms

The initial timeout is bounded by a minimum value, 500ms, and scales
linearly depending of the number of pairs on the way to be emited. The
same number of retransmissions than in the discovery state in used
during the connection check. The total time to wait for a pair to fail
is then RTO + 2*RTO + RTO = 4*RTO with 3 retransmissions.

On a typical laptop setup, with a wired and a wifi interface with
IPv4/IPv6 dual stack, a link-local and a link-global IPv6 address, a
couple a virtual addresses, a server-reflexive address, a turn relay
one, we end up with a total of 90 local candidates for 2 streams and 2
components each.  The connection checks list includes up to 200 pairs
when tcp pairs are discarded, with :

  <33 in-progress and waiting pairs in 50% cases (RTO = 660ms),
  <55 in-progress and waiting pairs in 90% cases (RTO = 1100ms),
  and up to 86 in-progres and waiting pairs (RTO = 1720ms)

The number of retransmission of 3 seems to be quite robust to handle
sporadic packets loss, if we consider for example a typical packet loss
frequency of 1% of the overall packets transmitted.

And a relatevely large initial timeout is interesting because it reduces
the overall network overhead caused by the stun requests and replies,
mesured around 3KB/s during a connection check with 4 components.

Finally, the total time to wait until all retransmissions have completed
and have timed out (2000ms with an initial timeout of 500ms and 3
retransmissions) gives a bound to the worst network latency we can
accept, when no packet is lost on the wire.