Summary:
A bunch of MPMCQueues linked together. Stage i produces exactly Ki (default 1)
outputs for each input. Ordering is preserved, even though stages might
produce (intermediate or final) results in parallel and in any order; we do
this by abusing the enqueueing mechanism in MPMCQueue. (Read the code for
details)

Test Plan: test added, more tests to be written before commit

Reviewed By: ngbronson@fb.com

FB internal diff: D892388