[processpool] schedule work to idle workers once per refresh cycle
[processpool] schedule work to idle workers once per refresh cycle

Reviewed By: mbouaziz

Differential Revision: D15373840

fbshipit-source-id: 77359a243

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