Using "eidx_key:" boolean prefix to limit results to a given
inbox, we can use ->ALL to emulate and replace per-Inbox
xap15/[0-9] search indices.
With this change, the presence of "extindex.all.topdir" in the
$PI_CONFIG will cause the WWW code to use that extindex and
ignore per-inbox Xapian DBs in xap15/[0-9].
Unfortunately IMAP search still requires old per-inbox indices,
for now. Mapping extindex Xapian docids to per-Inbox UIDs and
vice-versa is proving tricky. Fortunately, IMAP search is
rarely used and optional. The RFCs don't specify expensive
phrase search, either, so `indexlevel=medium' can be used in
per-inbox Xapian indices to save space.
For primarily WWW (and future JMAP) users; this should result in
significant disk space, FD, and page cache footprint savings for
large instances with many inboxes and many cross-posted
messages.