mozilla-releng / balrog / #4868

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feat: warm caches during app startup (#3500)

* feat: implement getReferencedReleases for SuperBlob

This will be useful when warming caches.

* refactor: add helper to find release regardless of which table it is in

* feat: warm caches during app startup

This code runs before the public app begins accepting requests, which should mean that we don't need to hit the database very much after beginning to serve requests. Doing this at start-up also means we fetch the releases sequentially, which should (slightly) spread the load out more.

I verified locally that the app is incapable of responding to requests until `public.py` finishes executing by adding a `time.sleep` to the end of it. While the sleep was running requests to the app hung. This means that `/__heartbeat__` requests won't complete successfully until caches have been warmed up.

Hopefully this will help with database load during scaling events. See: https://mozilla-hub.atlassian.net/browse/SVCSE-3425

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