mbox: disable "&t" on existing Xapian until full reindex
Expanding threads via over.sqlite3 for mbox.gz downloads without
Xapian effectively collapsing on the THREADID column leads to
repeated messages getting downloaded.
To avoid that situation, use a "has_threadid" Xapian metadata
flag that's only set on --reindex (and brand new Xapian DBs).
This allows admins to upgrade WWW or do --reindex in any order;
without worrying about users eating up bandwidth and CPU cycles.