conncheck: rework the stun requests ordering per timer tick

conncheck: rework the stun requests ordering per timer tick

With this patch, we merge the two variables stun_sent and
keep_timer_going. The three functions that are a possible source of a
new stun request returns a boolean value stating if a request has been
sent.  The semantic of keep_timer_going can now be deduced from
stun_sent and from the result of priv_conn_check_stream_nominate().

The trick that makes this merge possible is to repurpose the return
value of priv_conn_check_tick_stream(), because keep_timer_going set
when the conncheck list contains in-progress pairs in this function is
redundant with the same check later in function
priv_conn_check_tick_stream_nominate().