Per off site meeting:

• It should doable to use the gRPC C++ library to make outbound calls with the real stubs without doing any thread jumping. We might need a bit of guidance from @ctiller but it I think it could be done pretty quickly.
• Once this is done, we can stop using the "generic RPC" legacy code hacks, and actually use real stubs for ratelimit, auth, etc.
• We will eventually need to use the TSI library directly and have some type of TsiConnectionImpl class for upstream connections. This seems lower priority.