Heroku CI: Support building rails 6 with multiple databases on multiple nodes

We have multiple postgres databases (in different clouds in production) that we want to test against with Heroku CI with multiple nodes.

- Our primary database is set to use `heroku-postgresql:in-dyno` which works fine.
- A second database is running as `heroku-postgresql:standard-0`, and during `db:structure:load_if_sql` in the buildpack, whichever node arrives first loads the structure, and all later nodes spew ERRORs about the structure already existing.
+ On occassion, two nodes attempt to apply migrations simultaneously, causing one of them to error out the build with a `ActiveRecord::ConcurrentMigrationError` 😞.
+ Even with transactional tests, we sometimes fail tests that count database records. This seems like a bug elsewhere, but being able to provision a second in-dyno database (another db in the same postgresql server even) would solve this problem.

Start at the buildpack's db:structure:load_if_sql step and trace how Heroku CI nodes provision and initialize the second PostgreSQL database. Reproduce the multiple-node setup described here, then define completion as database initialization and migrations succeeding without concurrent-node errors.

Written by the indexing model from the issue text.