Revert "Update to standard library context package."

[dsnet]

Reverts #275

This is causing breakages for users. Will investigate more in-depth tomorrow.

[adamryman]

Could you mention what users this is causing breakages for? Is there a plan to switch to stdlib "context" as some point in the future?

[bradfitz]

Once Go 1.9 is out with type aliases, then we can do this without drama.

[jsha]

Bump! Go 1.9 is out now. :-)

[dsnet]

Unfortunately, the generation of context will not change until there are two releases of Go with type aliases. Thus, when Go1.10 is released, we'll make the change.

[domdoescode]

Is there a reason for waiting until two releases of Go with type aliases as opposed to versioning one with and one without? Just curious.

[justincormack]

1.10 is out now...

[bradfitz]

It'll be another 6 months or year due to App Engine stuff.