replace Xapian skeleton with SQLite overview DB

replace Xapian skeleton with SQLite overview DB

This ought to provide better performance and scalability
which is less dependent on inbox size.  Xapian does not
seem optimized for some queries used by the WWW homepage,
Atom feeds, XOVER and NEWNEWS NNTP commands.

This can actually make Xapian optional for NNTP usage,
and allow more functionality to work without Xapian
installed.

Indexing performance was extremely bad at first, but
DBI::Profile helped me optimize away problematic queries.