Add proper QRESYNC support on SELECT; use it for refresh_info

Add proper QRESYNC support on SELECT; use it for refresh_info

We aren't yet correctly updating the folder's modseq every time, so we're
often asking for more than we need. That's better than asking for _less_ than
we need though -- will go over that part very carefully in separate commit(s).

We still aren't handling uidvalidity either. RSN...