gerrit: switch to use std context package, require Go 1.7+

This ends support for Go 1.6.

Change-Id: If7765a15add556a5a7a881b24b1a38d96ff10841
Reviewed-on: https://go-review.googlesource.com/40938
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
6 files changed