Sphinx improvements, comment indexing

Performance improvements:

* Do not use AUTO_INCREMENT, use a better PRIMARY KEY that InnoDB that
reflects the data access patterns. (All accesses are either single-items
or groups-by-journal.)

* Use COMPRESS to save disk space.

* Don't copy entire journals every time something updates.

* Only copy once when imported -- at the end.

Functionality improvements:

* Add comment indexing and searching.

* Support delta indices for faster indexing cycles.

IMPORTANT: These changes are backwards-incompatible with search
installations. If you deploy this code, you will need to update
your database schema and delete the table and reimport (to get the
compression).