Time for partial download of nnimap messages again?

[Steinar Bang <sb@dod.no>]

Now that agent is always used by nnimap as a local cache, it may be
time to once more start looking at partial downloads of MIME messages?

Eg. downloading the text part, but leave that big videoclip on the
server, until we decide to look at it.

A quick revisit of the idea:
 - in the agent cache, replace the delayed parts with a
   message/external-body URL part
	<http://www.faqs.org/rfcs/rfc2017.html>
   where the URL is an IMAP URL:
	<http://www.faqs.org/rfcs/rfc2192.html>
 - when the user wishes to view or save the delayed part, it is
   downloaded from the server, and the message/external-body is
   replaced with the actual part

What Simon didn't like about this, IIRC, was that you changed the
message from the original.

But you don't, really.  The original message is kept in its priestine
state on the server.  What happens to the cached message is just an
internal Gnus implementation issue, IMO.