repo-updater: Support external_id migrations

Created by: tsenart

This change set makes it so we can support changes in what gets stored in external_id by the Syncer. Previously we enforced that all three of external_id, external_service_type and external_service_id would be set OR unset, together.

But in order to change what gets stored in the external_id column using the Syncer to essentially perform the migration, we need the external_service_id and external_service_type columns to remain intact, because we rely on external_service_type to filter by kind in DBStore.ListRepos.

If we'd unset all of the three columns together, we wouldn't list any repos, since they wouldn't be matched by the external_service_type column.

Precursor to solving #3465 because it needs to change what we store in the external_id columns of AWS Code Commit repos to a stable identifier.

Test plan: go test