*IMPORTANT* - bugfix sync_append_commit index breakage

[Rob Mueller]

>> I believe so.  If we also used it for appends, we wouldn't run into the
>> problem that Fastmail is seeing.  But, it may be too much of a
>> performance hit.

> I don't think that any of the code does the .NEW for appends because that
> makes append O(N) where N is the mailbox size, and that would suck for
> huge mailboxes.

Definitely. Rewriting the cyrus.index file for every message delivered to a 
mailbox would be horrible, especially when some users have 100,000+ messages 
in a mailbox.

> Speaking of which, is there any reason why cyrus.expunge isn't sorted
> by UID?  We have to rewrite the entire cyrus.index each time we do an
> expunge anyway.

I'm guessing it's because messages can be deleted + expunged in any order, 
so it's easy just to append the record for any message being expunged to the 
end of the cyrus.expunge file. If you had to keep it sorted, that would mean 
every time you expunged a message, you'd have to reread, resort, and rewrite 
the cyrus.expunge file. Generally the cyrus.expunge will be a lot smaller 
than the cyrus.index, but still annoying...

Rob