Use foreign keys more to avoid DB queries

[will-gant]

• A short explanation of the proposed change:

This PR ensures that where an id or guid of an associated resource is present in a DB row we've already retrieved, we get that from memory instead of making a separate query to the table for the associated resource.

Many database tables contain foreign keys linking them to associated resources - for example, the processes table has an app_guid column, and the service_instances table has space_id.

process.app.guid = two queries
process.app_guid = one query

• An explanation of the use cases your change solves

Optimization for speed

• Links to any other associated PRs

n/a

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests