La context (#521)

* middleware/proxy: give Exchange a context

Make context.Context the first paramater in the Exchange method.
This is inline with all other query functions.

* up the version

middleware/proxy/exchanger.go

@@ -1,6 +1,8 @@
 package proxy
 
 import (
+	"context"
+
 	"github.com/miekg/coredns/request"
 	"github.com/miekg/dns"
 )
@@ -8,7 +10,7 @@ import (
 // Exchanger is an interface that specifies a type implementing a DNS resolver that
 // can use whatever transport it likes.
 type Exchanger interface {
-	Exchange(addr string, state request.Request) (*dns.Msg, error)
+	Exchange(ctx context.Context, addr string, state request.Request) (*dns.Msg, error)
 	Protocol() string
 
 	OnStartup(*Proxy) error