Migrate mapping of changeset rewirer to DB
Created by: eseliger
This PR migrates the changesetRewirer from an encapsuled thing that both matched and in-place updated the changesets and changeset specs into two pieces:
- A store method to retrieve the mapping (which will be used in the resolver layer to expose these mappings as part of the delta API).
- A changesetRewirer that takes such a mapping and executes it.
I gave it a couple test-runs locally and it seemed to work fine. I'd love to get this in as early as possible so problems this might cause are detected as far from a release cut as possible.
Closes https://github.com/sourcegraph/sourcegraph/issues/16056