cancelCtx has an embedded sync.Mutex. This change causes an extra
allocation when creating a new timerCtx via WithDeadline, but that seems
minor when compared with better locking discipline. If it turns out to
be a problem, the whole cancelCtx struct can be flattened into timerCtx.

Addresses part of golang/go#14839.

Change-Id: Ie86ed1b63592b521aefde747d5fafcd49ac18178
Reviewed-on: https://go-review.googlesource.com/20840Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>