worker: Add common initialize methods for external services
Created by: efritz
This PR:
- adds external service initialization methods for:
- the frontend database
- the codeintel-db
- common code intelligence stores and clients (dbstore, lsifstore, gitserver client)
- sets up authz permissions
Nothing currently uses these, but there was some non-trivial memoization work I wanted to do separately before moving any actual behavior into this service.