conncheck: explicitely order the type of stun requests per timer tick

conncheck: explicitely order the type of stun requests per timer tick

With this patch, we try to make more explicit the process order between
the different types of stun requets, according that only one request is
sent per callback timer tick, ie every 20ms, to respect the stun pacing
of the spec. We implement the follow priority:

 * triggered checks
 * stun retransmissions
 * ordinary checks

In a concrete case, while a stream has stun requests related to
triggered checks to be sent, all other stun transactions are delayed to
the next timer ticks.

The goal of this patch is to make this priority explicit, and more
easily swappable if needed. Triggered checks have more probability to
succeed than stun retransmissions, this is the reason why they are
handled before. Ordinary checks on the contrary can be performed on a
lower priority basis, after all other stun requests.

The problem that can be sometime observed with a large number of stun
transactions is that stun retransmissions may suffer from a delay after
they have reached their deadline. This delay should remain small thanks
to the design of the initial retransmission timer (RTO), that takes into
account the overall number of scheduled stun requests. It allows all
stun requests to be sent and resent at a predefined "pacing" frequency
without much extra delay.

This ordering not perfect, because stun requests of a given type are
examinated per-stream, by looking at the first stream before the others,
so it introduces a natural priority for the first stream.