Remove boost dependency from folly/ProducerConsumerQueue.h
Remove boost dependency from folly/ProducerConsumerQueue.h

Summary: Boost is great, but c++11 incorporate some of their stuffs, and classes like ProducerConsumerQueue don't need it anymore.
Closes https://github.com/facebook/folly/pull/321

Reviewed By: fredemmott, yfeldblum

Differential Revision: D2519081

fb-gh-sync-id: b138a5af4662d1e7ef5e0823cf4001880ee02456

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