net/http: add a Request.Cancel channel.

net/http: add a Request.Cancel channel.

This allows for "race free" cancellation, in the sense discussed in
issue #11013: in contrast to Transport.CancelRequest, the cancellation
will not be lost if the user cancels before the request is put into the
transport's internal map.

Fixes #11013.

Change-Id: I0b5e7181231bdd65d900e343f764b4d1d7c422cd
Reviewed-on: https://go-review.googlesource.com/11601
Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>