1. Re-enable DTD validation once
   https://github.com/UweSchmidt/hxt/issues/15 is fixed.

2. Write a test for test/xml/Odds_XML-long-import.xml once it no
   longer takes 10 minutes to import (Postgres only?).

3. Write a test for the ON DELETE CASCADE behavior.

4. Add top-level message tables for all types so that we can link all
   records to a message and use the message's time stamp to (cascade)
   delete old data.

5. Drop the many-to-many relation between at least odds and
   odds_games, since duplicate games are highly unlikely and the table
   will grow without bound.