Since Go 1.7, "context" is a standard package, superceding the
"x/net/context". Since Go 1.9, the latter only provides type aliases
from the former. Therefore, it makes sense to switch to the standard
package, and the change is not disruptive in any sense.
This commit deals with a few cases where both packages happened to be
imported by the same source file. A choice between "context" and
"gocontext" was made for each file in order to minimize the patch.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
