priority CPU thread pool
Summary:
just extend CPUThreadPoolExecutor to use a queue that is itself composed of N mpmc queues, one per priority
the verbosity is starting to kill me, i had thought before of truncating Executor of all these pool types and now I'm definitely going to do that unless someone fights me.
Test Plan: added unit; maybe i'm not being clever enough as i couldn't think of many ways to test this reliably so there's just a basic preemption test
Reviewed By: davejwatson@fb.com
Subscribers: trunkagent, fugalh, njormrod, folly-diffs@, bmatheny
FB internal diff:
D1676452
Tasks:
5002392
Signature: t1:
1676452:
1416263990:
cdf5d44e4a50a6180ba547a3ed4c0c24d4ffdd8f