global: move to context (#1699)

* global: move to context

Move from golang.org/x/net/context to std lib's context.

Change done with:

for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done

* drop from dns.pb.go as well

plugin/cache/cache_test.go

@@ -4,7 +4,7 @@ import (
 	"testing"
 	"time"
 
-	"golang.org/x/net/context"
+	"context"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/pkg/response"