Implement and Document process for enforcing registry BBM

Context

The registry background migrations needs to implemented a way to ensure a batched background migration is completed much like how rails has the finalize a batched background migration process.

The registry should fail to serve traffic unless the migration to be finalized/enforced was completed prior.

Task

• The registry needs to fail to start (with information on the failure in the logs) when a BBM is enforced but is not complete. A BBM may be enforced by:

  • Having a dependency on a new schema migration (for example if a new database schema migration requires that a background migration be completed before it can run)
  • Programatically in the code (needed when we want to stop the registry from starting because the version requires a background migration that may not have been completed)

• Update the Batched background migration documents making sure it aligns with the current design