Refactor sharedmem API: tweak cache functor signature
Refactor sharedmem API: tweak cache functor signature

Summary:
We only use the key type from the UserKeyType argument, so just combine with the
existing cache config argument.

Reviewed By: jbrown215

Differential Revision: D24572001

fbshipit-source-id: aacfcfc7e7076c42b4bb8b1817123799b285da92

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