Remove '0000-00-00 00:00:00' datetimes.

Previous iPeer versions sometimes use '0000-00-00 00:00:00' for datetime
fields. Unfortunately, newer MySQL versions now rejects it as an invalid
date, so we'll have to use NULL for empty datetimes, I guess.
Remove setting AUTO_INCREMENT's starting index.

Also removed some of the manual setting IDs, since MySQL seems to be
able to automatically figure those out anyways.