Run golint and go vet (#276)

Cleanup the errors and removed deadcode along the way. The leaves
some error laying around, mostly about commenting exported identifier.
We should look hard if those really are needed.

middleware/metrics/metrics.go

@@ -30,7 +30,7 @@ type Metrics struct {
 	Addr      string
 	ln        net.Listener
 	mux       *http.ServeMux
-	Once      sync.Once
+	Once      *sync.Once
 	ZoneNames []string
 }