If the parent context passed to WithCancel or WithTimeout
is a known context implementation (one created by this package),
we attach the child to the parent by editing data structures directly;
otherwise, for unknown parent implementations, we make a
goroutine that watches for the parent to finish and propagates
the cancellation.

A common problem with this scheme, before this CL, is that
users who write custom context implementations to manage
their value sets cause WithCancel/WithTimeout to start
goroutines that would have not been started before.

This CL changes the way we map a parent context back to the
underlying data structure. Instead of walking up through
known context implementations to reach the *cancelCtx,
we look up parent.Value(&cancelCtxKey) to return the
innermost *cancelCtx, which we use if it matches parent.Done().

This way, a custom context implementation wrapping a
*cancelCtx but not changing Done-ness (and not refusing
to return wrapped keys) will not require a goroutine anymore
in WithCancel/WithTimeout.

For #28728.

Change-Id: Idba2f435c81b19fe38d0dbf308458ca87c7381e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/196521Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>