middleware/proxy: absorb httpproxy (#481)

* middleware/proxy: absorb httpproxy

Move the httproxy into proxy. This adds and Exchanger interface which
is used to exchange the messages with the upstream.

The https_google upstream will re-resolve itself and update the upstream
hosts used every 300s.

* Remove and add TODO

middleware/proxy/exchanger.go

@@ -8,11 +8,9 @@ import (
 // Exchanger is an interface that specifies a type implementing a DNS resolver that
 // can use whatever transport it likes.
 type Exchanger interface {
-	Exchange(request.Request) (*dns.Msg, error)
-	SetUpstream(Upstream) error // (Re)set the upstream
-	OnStartup() error
-	OnShutdown() error
-	Protocol() protocol
-}
+	Exchange(addr string, state request.Request) (*dns.Msg, error)
+	Protocol() string
 
-type protocol string
+	OnStartup(*Proxy) error
+	OnShutdown(*Proxy) error
+}