The existing string -> number date range Xapian query is good
enough, and having too much flexibility is probably bad for
caching (as well as increasing our attack surface, because
parsing queries is tricky).
Tags-as-skiplists are probably not worth the effort given
Xapian, and we may have to import old messages after-the-fact,
anyways, and message delivery for mirrors is never orderly.
Other items are all done and need to be maintained (like the
search engine docs for the mairix-compatibility features that
just got pushed out)