Switch from x/net/context to context

Since Go 1.7, context is a standard package, superceding the
"x/net/context". Since Go 1.9, the latter only provides a few type
aliases from the former. Therefore, it makes sense to switch to the
standard package.

This commit was generated by the following script (with a couple of
minor fixups to remove extra changes done by goimports):

	#!/bin/bash

	if [ $# -ge 1 ]; then
		FILES=$*
	else
		FILES=$(git ls-files \*.go | grep -vF ".pb.go" | grep -v
	^vendor/)
	fi

	for f in $FILES; do
		printf .
		sed -i -e 's|"golang.org/x/net/context"$|"context"|' $f
		goimports -w $f
		awk '	/^$/ {e=1; next;}
			/[[:space:]]"context"$/ {e=0;}
			{if (e) {print ""; e=0}; print;}' < $f > $f.new && \
				mv $f.new $f
		goimports -w $f
	done
	echo

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

services/content/service.go

@@ -17,6 +17,7 @@
 package content
 
 import (
+	"context"
 	"io"
 	"sync"
 
@@ -30,7 +31,6 @@ import (
 	digest "github.com/opencontainers/go-digest"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
-	"golang.org/x/net/context"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"