mcrouter/lib/network/McClientRequestContext.cpp: avoid a shadowing warning (trivial)
mcrouter/lib/network/McClientRequestContext.cpp: avoid a shadowing warning (trivial)

Summary:
The "id" parameter in a lambda was shadowed by an outer, local "id".
Rename the inner one to "i".

Reviewed By: yfeldblum

Differential Revision: D3993634

fbshipit-source-id: d049aa15a92fab453349d0a05e3581692cb2eb9b

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