Implement windowing for IMAP UID FETCH

Our previous implementation of UID FETCH didn't ever take into account
maximum command line lengths. When fetching, say 800 messages from a
GMail IMAP server, we could easily overflow the max line length leading
to a fetch that didn't get all the messages we wanted to and was
truncated before the description of which fields we want. That caused
K-9 to fetch complete messages, exhaust memory and ultimately fail,
even when we were just trying to get message lengths.

An equivalent fix needs to be made to seach by UID.