middleware/proxy: make it scale (#287)

* middleware/proxy

Use connection pooling for communicating with an upstream, instead of
opening a new socket every time.

This makes the proxy more efficient and allowed for some cleanups.

* Some cleanups

* Some fixes

* more

* Kill pool

* Add nil check

* remove pool

middleware/proxy/setup.go

@@ -20,7 +20,7 @@ func setup(c *caddy.Controller) error {
 		return middleware.Error("proxy", err)
 	}
 	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
-		return Proxy{Next: next, Client: Clients(), Upstreams: upstreams}
+		return Proxy{Next: next, Client: NewClient(), Upstreams: upstreams}
 	})
 
 	return nil