This large diff represents a growth of the architecture to persist IMAP data as its downloaded. When listing folders, a local database is consulted first to immediately feed to the caller. In the background, network calls fetch the "real" list. The two are collated for differences which are reported to the caller via signals, who are then responsible for updating the user interface appropriately. No other synchronization work is represented in this diff.
Note that this breaks functionality: when a folder is selected, no messages appear in the message list. Fixing this requires more work, and this patch was already large enough. It's ticketed here: #3741
