[prover] Sort proving jobs by discrete priorities

[spalladino]

In #7844 we added an epochNumber for sorting jobs. However, this makes it difficult to implement with a fifo queue with no support for priorities. Instead, we could use a discrete set of priorities (eg level in the proof tree) to sort jobs, so this can be implemented with multiple fifo queues.

Alternatively, we can use something that is not a queue for sorting jobs, such as a db, given we won't have such a large tx throughput. If we do so, and we enshrine epoch numbers as a priority, then this issue turns into addressing @alexghr's comments on the original PR to make this priority mandatory and part of the proving job itself.

Needs further discussion.