Instantly send result to client

Depends on #29.

This PR makes sure that as soon as the Client sends an ExecuteRequest, the ClientHandler will respond with a FetchResponse if the result is already known.

It improves the efficiency a bit, because it saves the Client a round trip if the result is known. On the other hand, it sightly changes the contract between Client and ClientHandler.

Since the protocol is never offically documented, and AlphaRPC does not (yet) have a policy of supporting third-pary clients, I think it is okay to merge it though.