This patch aims to implement more closely the algorithm described
in RFC 5245 indicating how pairs are transitionned from state Frozen
to Waiting. This is described in 7.1.3.2 when a check succeeded, and
correspond to modifications in function priv_conn_check_unfreeze_related().
This is also described in 5.7.4 when defining the initial state of the
pairs in a conncheck, and correspond to modifications in function
priv_conn_check_unfreeze_next().

This patch introduces the notion of active and frozen check list. It
allows us to define the timer restranmission delay as described in 16.1.

Another modification in priv_conn_check_tick_unlocked() is that every
stream in handled consecutively, and in an independant way. The pacing
was previously of a single STUN request emitted per callback, it is now
of a triggered check per callback OR a single STUN per callback AND per
stream per callback.

The description of ordinary checks per stream in 5.8 is detailled in
function priv_conn_check_tick_stream(), and a remaining of the code
used to nominate a pair by the controlling agent is put in a dedicated
function priv_conn_check_tick_stream_nominate()

Differential Revision: https://phabricator.freedesktop.org/D813