Cache parsed date, subject, author, and recipient in memory in IMAIL.

Cache parsed date, subject, author, and recipient in memory in IMAIL.

This is not really the right thing, but it substantially speeds up
sorting, and slightly speeds up summarization, without requiring the
memory overhead of keeping whole headers strongly in memory.

What IMAIL should really do is

(1) store headers (and bodies and body structures and so on) strongly
in memory, and use a secondary GC daemon to discard them when space is
short; and

(2) use a generic, compact, on-disk cache, for every folder, of the
important information for each message: date, subject, author,
recipient, message-id, thread-id.