[core] Static Priority scheduling (4/N)

[dayshah]

Why are these changes needed?

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[dayshah]

@jjyao don't need a real code review rn, but can you do a quick logical look over where I changed to account for priority just to make sure that there's no other spots where priority should be accounted for. Any ideas to minimize the time between:
higher priority tasks submitted -> higher priority tasks start superceding lower priority?

Can also look at the 1 -> 2 -> 3 -> 4 diffs here https://github.com/dayshah/ray/pulls for easier to see the four sets of changes.