[core] Add preemption logic

[asm582]

We need the ability to preempt AWs given a list of AWs by other libraries like the quota management system

[tardieu]

Agreed. The challenge here is that preemptable appwrappers right now are a simple function of priority. As a result, we can compute the preemptable appwrappers once for all pending appwwappers at the same priority level. With quotas, preemptable appwrappers become dependent of the candidate appwrapper for dispatch. So this computation is much more expensive. We'll have to see how much and this may make some form caching necessary if possible.