Fix data race in ProcessPool
Fix data race in ProcessPool

Summary:
This diff fixes a data race in ProcessPool: out channel flush was
outside of the critical section.

Reviewed By: ezgicicek

Differential Revision: D17853991

fbshipit-source-id: ac0fd2a69

jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer