codeintel: Isolate background processes

Created by: efritz

We have had issues in the past with background processes running periodically but consuming too many resources. For code intel, we have several background processes running in the frontend instances, which could be moved to a separate container to better control resource consumption:

• custodial tasks (removing expired db records)
• auto-indexing tasks (adding records to the lsif_indexes queue)
• commit graph recalculation