Since we only query the SQLite over DB for OVER/XOVER; do not
need to waste space storing fields To/Cc/:bytes/:lines or the
XNUM term. We only use From/Subject/References/Message-ID/:blob
in various places of the PSGI code.
For reindexing, we will take advantage of docid stability
in "xapian-compact --no-renumber" to ensure duplicates do not
show up in search results. Since the PSGI interface is the
only consumer of Xapian at the moment, it has no need to
search based on NNTP article number.