Adding checksum into the DB

This adds checksum support to the DB. Specifically:
- New table `checksum_algorithm` with two columns: `checksum_algorithm_id` and `name` to store the algorithms used for the checksum generation.
- Two new columns on table `filepath`: `checksum` (stores the actual checksum) and `checksum_algorithm_id` (foreign key to `checksum_algorithm` table).

The files `initialize.sql` and `populate_test_db.sql` have been updated to reflect this changes.

This should be fast to review. @squirrelo, since you've been working a lot with dbschema, it would be great if you can take a look and make sure that I've updated everything correctly. This is blocking further development on any object that modifies the `filepath` table.