hashmap changes to save CPU and LLC misses
hashmap changes to save CPU and LLC misses

Summary: as per title

Reviewed By: binliu19

Differential Revision: D7536437

fbshipit-source-id: 3e561733d78225fc4bcfe2ed6e1b9d6a08ab465d

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