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/etcd/setup.go

@@ -9,9 +9,10 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/upstream"
 	"github.com/coredns/coredns/plugin/proxy"
 
+	"context"
+
 	etcdc "github.com/coreos/etcd/client"
 	"github.com/mholt/caddy"
-	"golang.org/x/net/context"
 )
 
 func init() {