rate: use standard library context package

Fixes golang/go#16745

Change-Id: I34f87a2ae570c2bb0ce9ccb72fc5be0d34266e49
Reviewed-on: https://go-review.googlesource.com/41194
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dan Peterson <dpiddy@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>