a non-concurrent token-bucket variation
a non-concurrent token-bucket variation

Summary: Adds a policy around whether token-bucket is used only non-concurrently, in which case compare-exchanges can be optimized to stores.

Differential Revision: D28230482

fbshipit-source-id: 2c00797dc943e9ea8985737a8c7dda059a195cfe

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly