Enhance preloading of folder outlines for IMAP folders, and implement

Enhance preloading of folder outlines for IMAP folders, and implement
caching of entire folders' contents.

PRELOAD-FOLDER-OUTLINES on IMAP folders will now be much cleverer
(read: not stupid) about what items to fetch for what messages, so that
we fetch only what we need from each message.  This means that summary
buffer generation will now spend much less time (i.e. as much time as
it did six months ago) fetching message items.

New generic procedure CACHE-FOLDER-CONTENTS works similarly to
PRELOAD-FOLDER-OUTLINES, but also fetches requested body parts of
messages.

New Edwin command IMAIL-CACHE uses CACHE-FOLDER-CONTENTS to fill the
cache of the selected folder.  The front end's generic procedure
WALK-MIME-MESSAGE-PART is now a little more general, so that it can be
used to work together with with CACHE-FOLDER-CONTENTS.