Optional extra checks for content of "base" schema #161

When troubleshooting sync issues, it would be useful to have an option to do extra checks of the "base" schema:

• at the end of each sync, db-sync could create some kind of checksum for each table in the base schema
• when syncing again, db-sync could verify that the base schema has not changed since the last run

(Currently we don't have that and when there are issues, it's hard to tell whether it is because something got changed in the database by some other party, or because there's a bug somewhere)