Fix sigsev error happening due to reference to destructed key in CarbonLookasideRoute
Fix sigsev error happening due to reference to destructed key in CarbonLookasideRoute

Summary: Refer to stack trace of crash in HHVM CarbonLookasideRoute P507513785. The issue seems to be raised due to `key` being destructed here and the construction of `McSetRequest` reference a freed memory, leading to throwing of exception in GuardAllocator.

Reviewed By: disylh

Differential Revision: D37087468

fbshipit-source-id: 8c673fee679c57fbb189d5b85f2f953fa902bae0

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