Add HyperSQL as an alternative DB library for testing

This will help to ensure our DB code stays portable, in case we need to migrate away from H2.

This looks like a big patch, but most of the changes are:

• Refactoring tests so we can run them with H2 and HyperSQL
• Making placeholder type names more obvious in SQL by adding an underscore
• Renaming a couple of columns whose names turned out to be SQL keywords :-/