Implement incremental processing of IMAP responses, so that long lists

Implement incremental processing of IMAP responses, so that long lists
of large responses, such as all the header fields in a large folder,
will not be queued up to exhaust the heap space.  Exploit this when
preloading folder outlines, and don't fetch the header fields when
only the size is needed.  (Preloading folder outlines no longer even
fetches the size -- but to do this right requires a lot more work and
would probably make preloading the outlines slower anyway, and I can
live with slightly more network-intensive summary buffer generation
for folders never before summarized.)  Store messages' header fields
and envelopes only weakly in memory to conserve space.