Implement work stealing #373
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The worker scheduler would previously assume that it was fine to schedule infinite amounts of work onto the same processor at once, which is only efficient when tasks do lots of `yield`ing. Because most tasks do not actually exhibit low occupancy, we want to teach at least the worker scheduler to limit its eagerness when executing high-occupancy tasks. This commit teaches `@spawn` and the worker scheduler about a new `occupancy` task option, which (on the user side) is a value between 0 and 1 which approximates how fully the task occupies the processor. If the occupancy is 0.2, then 5 such tasks can execute concurrently and fully occupy the processor. Processors now operate primarily from a single controlling task per processor, and work is executed in a lowest-occupancy-first manner to attempt to maximize throughput. With processors using occupancy estimates to limit oversubscription, it's now quite easy for tasks to become starved for work. This commit also adds work-stealing logic to each processor, allowing a starved processor to steal scope-compatible tasks from other busy processors. Processors will be able to steal so long as they are not fully occupied.
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Adds logic to do work stealing within and across workers, which can help prevent workload imbalance and processor starvation.
Todo:
Sch.Doorbell(Base.Event)Add testsExisting tests should be sufficient