Don't use standard lib context package (#1468)

With Go 1.9 you *can* include the std lib's context package and nothing
breaks. However we never officially made the move (and grpc also doesn't
ues the std lib's one).

Standardize all plugins on using the extern context package.

Fixes #1466

plugin/proxy/google.go

@@ -1,7 +1,6 @@
 package proxy
 
 import (
-	"context"
 	"crypto/tls"
 	"encoding/json"
 	"fmt"
@@ -16,6 +15,7 @@ import (
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
+	"golang.org/x/net/context"
 )
 
 type google struct {