ClientUnaryReactor is a reactor-style interface for a unary RPC.
This is not a common way of invoking a unary RPC. In practice, this option should be used only if the unary RPC wants to receive initial metadata without waiting for the response to complete. Most deployments of RPC systems do not use this option, but it is needed for generality. All public methods behave as in ClientBidiReactor. StartCall is included for consistency with the other reactor flavors: even though there are no StartRead or StartWrite operations to queue before the call (that is part of the unary call itself) and there is no reactor object being created as a result of this call, we keep a consistent 2-phase initiation API among all the reactor flavors.
