RFC 697: Ensure minor version checks are respected in the migrator

Created by: efritz

Until we completely support multiple version upgrades, we need to ensure that the migrator will fail-fast (and not update the database schema) while this check would fail.

As it stands, we have a nasty situation that can be caused by jumping up multiple versions. The schema will (possibly) be updated, but the frontend will refuse to start. Downgrades are difficult in this situation as we can't reliably revert to the previous version as the schema has already been updated.

We should copy the condition that checks the minor version distance into the migrator so that the error message comes from the migrator and not only from the frontend. When this condition is detected we should do nothing to the state of the instance's data.

We can remove both of these checks once we support multiple version upgrades.