net/http: make Transport.CancelRequest somehow work against http2

[bradfitz]

https://golang.org/cl/18873 documented loudly that CancelRequest doesn't work against HTTP/2 servers, but perhaps we can make it work anyway.

More background: #13540 ("x/net/http2: decide what to do about Transport.CancelRequest"), which made http.Client stop using CancelRequest itself.

Really, people should just stop using CancelRequest. But in many cases we created the Request, so we could also own the cancel channel and use it.

[bradfitz]

If we lived with this in Go 1.6, we can live with it in Go 1.7 and beyond. CancelRequest should just remain deprecated.