This modifies commit 8f1f615e. It is better focused to update the
selected pair just after its nominated flag has been set. We also keep
the code homogeneous with other places, where the call to
priv_update_selected_pair() immediately follows the setting of
pair->nominated. Moreover in priv_update_check_list_state_for_ready(),
we would call priv_update_selected_pair() more times that necessary when
iterating on all nominated pairs.

Differential Revision: https://phabricator.freedesktop.org/D1125