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.

Start at the ntci::Proactor::cancel and ntci::ProactorSocket interfaces and trace their implementations to understand how pending operations are canceled. Define completion as invoking ntci::ProactorSocket::processCanceled() only after all pending operations have been canceled, then verify the affected proactor and socket behavior across implementations.

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