RFC 235: Update bundle manager to read from Postgres

Created by: efritz

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

Update the bundle manager database layer to read from postgres, then fall back to data on disk. This should be done before actually writing to Postgres so that we can ensure that someone is reading the new data instead of blackholing writes.

This issue must follow https://github.com/sourcegraph/sourcegraph/issues/13884.