Enhance ntci::Proactor::cancel to call back to the user when all operations are canceled

The signature of `ntci::Proactor::cancel()` implies that is synchronous, but it is likely that for some implementations cancelation must be implemented asynchronously. Enhance the contract of `ntci::Proactor::cancel` to call a new function, `ntci::ProactorSocket::processCanceled()`, when all pending operations are canceled.