Improve imap fetch handler to accept an initial UID. (closes #3969)

Improve imap fetch handler to accept an initial UID. (closes #3969)

Gmail sends flag updates with a UID before the FLAGS.  The handler was
very simple, and so ignored the flag update in that case.

Pull the code from msg_parse_fetch(), paring down to just UID and
FLAGS.  This will handle arbitrary order data items.