Fix drop probability for routers other than memcache
Fix drop probability for routers other than memcache

Summary:
Drop probablity was wrong for all routers except Memcache.
It was ignoring MemoryController and it was doing redundant extra checks.

Differential Revision: D5535023

fbshipit-source-id: 938ad76d486c088bff2fb8bec507594f94cc2e78

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