conncheck: make the stopping criterion a bit more clear

This patch should not change the logic of the selection of the pair for nomination, just make the code a bit more simple to read, by storing the previously selected pair for nomination in the stream structure (for pair comparison among other components) and in the agent structure (for comparison among other streams), instead of recomputing it.

I choose the term "nominee" pair to designate these pairs to be rechecked with the use-candidate stun attribute set. This term will not clash with "selected" pair and "nominated" pair, which has other meanings in other parts of the code. These pairs don't have the "p->nominated" flag set yet, but have the "p->use_candidate_on_next_check" flag.

I also came back to a gboolean variable for the stopping criterion instead of a gchar*. Should these nice_debug() be removed, the code wouldn't be cluttered with static strings.