htsn-import-0.0.1: doc/TODO

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 a test for inserting duplicate messages.