Document design for RPC Client

[gregmedd]

How are responses arriving at the shared callback mapped to the correct promise? How will promises be broken when it is time to shut down?

Chris recommends that this be assigned to Uzi

[gregmedd]

For now, we want the simplest implementation of RPC Client / Server. So long as we can maintain backwards compatibility on the interface, we can release enhancements down the road (e.g. adding a thread pool)