Use std::atomic instead of volatile and custom barriers in WorkerPool
Use std::atomic instead of volatile and custom barriers in WorkerPool

Reviewed By: Maratyszcza

Differential Revision: D5745673

fbshipit-source-id: fcaabe941847e58624c8e87d27ccc607dc74e27f

sf-wind/caffe2,sf-wind/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2,sf-wind/caffe2,caffe2/caffe2,xzturn/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2