Fix parsing of IMAP folder names (#4830).
Merge remote-tracking branch 'drebs/bug/4830_handle-unicode-in-folder-names' into develop
defer costly operations
Move flags and other metadata to a separate doc.

This change will allow for quicker access times, and smaller
syncs since the fields that change more often will fall in a
pretty small document. For the big raw message, we only need
to sync once.

Also, implemented multipart interface for messages.
This will need additional migration helper in --repair-mailboxes.
remove lingering uppercase
inlineCallbacks all the things!