Store MODSEQ in the header cache, and use that to perform a "FETCH
CHANGEDSINCE" for header updates when initially downloading messages.

Further improvements could be made to add support when syncing.
Handling MODSEQ for FLAG updates while the mailbox is open would be
complicated by the fact that Mutt supports locally modified headers,
so we couldn't accept the new (or subsequent) MODSEQ.

However, this initial step may at least provide some benefit when
opening the mailbox, which is generally the most time and data
intensive.