Remove obsolete comments

This definitely should be UID SEARCH and UID FETCH, as we use the UIDs
to restrict the number of messages we fetch.
Compress ranges when fetching new messages
Make use of CONDSTORE capability when downloading flags

If the server supports CONDSTORE, use SELECT with (CONDSTORE) as a
CONDSTORE-enabling command.  Remember HIGHESTMODSEQ response, and use it
when fetching flags.