Add disconnect flag to OkHttpClient.cancel

Before this change, cancel would apply either before the request was made, or after the response was received.  This change allows it to apply to an in-flight request.

DO NOT MERGE, yet.
I've not written tests in okhttp, yet, as I want to ensure we are on the same page with implementation.  That said, the changes here do in fact cancel an in-flight HTTP/2 request, verified in a downstream project.