Ensure that async dml operations are run before merging

When adding an async index or foreign key, we should ensure that the they are run before being merged. At a minimum, they should be executed against a test database, but ideally they'd run against database lab. That said, I'm not sure if gitlab-com-database-testing is the right place for them either since they can take such a long time.

Deliverables

• Async index and foreign key creation executed somehow prior to merge
• Consider if we want this in gitlab-com-database-testing

Ideas

• Maybe we could just execute them synchronously in dev? The sync helpers are all idempotent, so it would actually be fine to have it include them in the structure.
• Maybe we want to make them run synchrounously in gitlab-com-database-testing, but we should look at some of the timings and decide if that's actually a good idea. I worry that those jobs could take a very long time.