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...
(cherry picked from commit 7dcedc3e)

(Disabled the UI configuration option for it, since it's a new string)