Instantly send result to client if it is available.

If the ClientHandler knows the result for the request that the Client
wants to execute, it now sends it back immediately.

This improves efficiency, but slightly changes the (nondocumented)
contract between the Client and the ClientHandler. It does not change
the actual behaviour of the Client, though.

Signed-off-by: Jacob Kiers <jacob@alphacomm.nl>