Use `boost::thread` in `PriorityThreadPool`
Use `boost::thread` in `PriorityThreadPool`

Summary: (Temporarily?) use `boost::thread` in the `PriorityThreadPool`, because for some certain large code we need the stack space in the inliner.

Reviewed By: ssj933

Differential Revision: D24385820

fbshipit-source-id: 0d102e7fb83b2e527ba35822fa4af35cdc9222f4

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