Make updateCounter and curWeightedValue more accurate
Make updateCounter and curWeightedValue more accurate

Summary: Reorder the code and make the value read and computation closer.

Differential Revision: D3789933

fbshipit-source-id: 39006888f7408e58c64f2c93baacb3cb5a2bd70e

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter