Cancel and re-schedule low-priority task if high-priority task is scheduled

[ahoppen]

When TaskScheduler has all execution slots filled up with low priority tasks and a high-priority task is scheduled, we should cancel and reschedule the newest low-priority task (newest because it has probably made the least progress that we would cancel).

rdar://127474310

[ahoppen]

Decided not to do this because: Priority elevation of currently executing tasks is done asynchronously by polling in withTaskPriorityChangedHandler. We could thus reasonably get into the following scenario: All execution slots in the AsyncQueue are filled up with low-priority tasks. We now await all of them from a high-priority task (eg. because we want to ensure that the index is up-to-date). At the same time, we schedule a new high-priority task. If we implemented this, then we would cancel and reschedule one of the low-priority tasks only because its priority elevation hasn’t been propagated by priority polling. And that would be sub-optimal because it duplicates work. I think it’s better to just let the low-priority tasks finish.