Add support for LoadBalancerRouter for MemcacheRouterInfo
Add support for LoadBalancerRouter for MemcacheRouterInfo

Summary:
Add support for LoadBalancerRoute as a "standalone route handle" (before it
could just be used a "hash function" of PoolRoute).

Reviewed By: jmswen

Differential Revision: D7063265

fbshipit-source-id: fd3c057cae0854baeb6eb55c632f6137807de0d1

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