allow customizing processing order

[TC5027]

Fix #985

[bgentry]

Hi, I'm not sure this really does what you think it does. AFAICT the only actual change here is that you're now querying based on scheduled_at DESC instead of scheduled_at ASC. That's not really "last-in first-out", and it's conceptually not aligned with how we use scheduled_at for various things.

Let's keep the discussion about the use case on #985 but in its current form this PR isn't something we can accept because it'll break a bunch of things and add a sizable maintenance burden, and without more detail I'm not sure if it even makes sense as a way to solve the underlying use case.

[TC5027]

I can review my implementation to avoid duplicating the queries if that's a concern, I did so because from my knowledge there is no other way to do so for sqlite
Also if you want me to add tests i can probably do.
Why reasoning on scheduled_at is wrong ?