migration: Make store initialization intelligent

Created by: efritz

Fixes #39579 (closed).

This PR makes the migration store backfill migration logs on startup. This has two effects:

• We can read from the schema_versions in older versions of Sourcegraph to determine which version they've migrated up to so far prior to an upgrade.
• We can backfill the tail of the migration graph if a new instance starts prior to the first squash. This will help upgrades in the future so that we don't try to re-run migrations that were logically applied on the instance's first boot, but not explicitly listed in the migration logs of the first run.

Test plan

Additional unit tests in this PR.