New implementation of folder orders that uses AVL trees instead of

New implementation of folder orders that uses AVL trees instead of
sorted vectors to store the order of messages.  Modifications to the
folder are now reflected in incremental updates to the folder order by
AVL insertions, instead of rebuilding the order vector for every
modification to the folder.  Also, the key by which the message is
sorted is now computed once per message in the order record, instead
of every time that the message comparison predicate is called.