codeintel: Extract generic worker process
Created by: efritz
This PR generalizes and extracts the logic from the codeintel indexer and worker processes that pull work from postgres and process it. This PR closes https://github.com/sourcegraph/sourcegraph/issues/11786.
Major changes:
- Please see all changes in internal/workerutil. This holds the new Worker and Resetter implementations. This also modifies the Store slightly so that it has a mockable interface and has the MarkComplete/MarkErrored methods.
- Code intel can (quickly) vet the changes in the worker and indexer services, which is mostly deleting code that's not implemented in the internal package.