Pass RequestCallback to FaultyPG RPC agent
Summary: This is needed to avoid FaultyPG from including and depending on RequestCallbackImpl, which is Python-only. The other RPC agents accept an explicit (upcast) pointer as an argument, and we can do the same for FaultyPG.
Test Plan: Later in the stack.
Reviewed By: mrshenli
Differential Revision: D29132955
fbshipit-source-id: bb7554b84bcbf39750af637e6480515ac8b92b86
