With this patch, we update the state of the pair to waiting when
it is put in the triggered check queue. We also take care to call
priv_schedule_triggered_check() before priv_mark_pair_nominated()
so a pair to be rechecked and put on the triggered check queue
will have a unique state to be tested in the following call to
priv_mark_pair_nominated() when evaluating its nomination attributes.

Differential Revision: https://phabricator.freedesktop.org/D883