context: use Go 1.9 type alias for CancelFunc and Context

Since Go 1.7 started including the context package in the core
distribution, this package has been forwarding to the core package's
implementation to the best of its ability. Since type aliases are due
to be included in the Go 1.9 release, use them to forward more

Change-Id: I97b92cd34b3f4426f565bb3d115f119d3c83a1dd
Run-TryBot: Emmanuel Odeke <>
TryBot-Result: Gobot Gobot <>
Reviewed-by: Brad Fitzpatrick <>
3 files changed