* remove golang.org/x/net dependency

The golang.org/x/net/context package is an alias for stdlib context
since go1.7 (see https://go-review.googlesource.com/72570

)

This patch replaces all uses of it for stdlib context.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

* rename App Engine context type to remove import aliases

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

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Signed-off-by: Sebastiaan van Stijn <github@gone.nl>