Instead of optimizing our own performance, this optimizes
our data to reduce work done by the MUA consumer.
Maildir and mbox destinations no longer support any notion of
the IMAP \Recent flag. JMAP has no functioning \Recent
equivalent, and neither do we.
In practice, having MUAs (e.g. mutt) clear the \Recent flag when
committing changes to the mbox is expensive: it creates a
rename(2) storm with Maildir and overwrites the entire mbox.
For mboxcl2 (and mboxcl), we'll further optimize mutt behavior
by setting the Lines: header in addition to Content-Length.
With these changes, mutt exits instantaneously on mboxcl2,
mboxcl, and Maildirs generated by "lei q".