RFC 235: Migrate SQLite data to Postgres

Created by: efritz

This is part of RFC 235: Move LSIF data to Postgres.

Write a migration routine that will start on bundle manager startup (and run as long as necessary) that reads SQLite bundles on disk that are not yet present in Postgres and issue writes for the same data. After a SQLite bundle's data has been written to Postgres, it can be moved into a new directory. This new directory can serve as a backup directory. The operator can decide if they need to keep this data in case of a version downgrade, or explicitly delete it otherwise.

This issue must follow https://github.com/sourcegraph/sourcegraph/issues/13887.