api to access cyrus header/index files

[Rob Mueller]

> If you're using v2.2.3, its fairly easy.  In mailbox.h, bump
> MAILBOX_CACHE_MINOR_VERSION to 3 and in mailbox.c add an entry like
> {"Xmyheader", 3} to the mailbox_cache_headers array.  Any new messages
> added to the mailbox will have this header cached.  If you want all of
> the messages to have this header cached, then you'll have to reconstruct
> the mailbox.

It would be extra nice if one day this stuff was configurable for your
installation, rather than having to patch the source. For instance, we add
X-Spam-score: and X-Spam-hits: headers and one day I'd like to add code to
display these in our web-interface. To keep the performance reasonable
though, I'll have to patch so that it caches these headers. Every extra
patch is annoying to have to maintain in the long run...

Rob