nnmaildir now has persistent article numbers

[prj@po.cwru.edu (Paul Jarc)]

And it's (somewhat) concurrency-friendly, too: you can run two Gnusae
running nnmaildir-request-scan on the same maildir at the same time,
and they won't assign the same article number to two different
messages.  In the spirit of maildir, this is done without needing file
locks.  (However, it could happen that a single message gets two
numbers assigned to it.  Only one number will be stored in the
maildir, but the two Gnusae will disagree for as long as they re both
running.  So don't do that after all.)

Article numbers are stored in .nnmaildir/nov/*.  So caching and
agentizing should work now.  In the past, you could remove the
.nnmaildir/nov/... file to force regeneration of NOV data (although it
should always happen automatically when needed).  Beware that this
will now also cause the message to get a new article number - unless
you do it while Gnus is running and do M-g on the group afterwards, in
which case the old number will be remembered and stored back in the
new NOV file.


paul