etcdmain, tcpproxy: srv-priority policy

Adds DNS SRV weighting and priorities to gateway.

Partially addresses #4378

proxy/tcpproxy/userspace_test.go

@@ -42,9 +42,11 @@ func TestUserspaceProxy(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	var port uint16
+	fmt.Sscanf(u.Port(), "%d", &port)
 	p := TCPProxy{
 		Listener:  l,
-		Endpoints: []string{u.Host},
+		Endpoints: []*net.SRV{{Target: u.Hostname(), Port: port}},
 	}
 	go p.Run()
 	defer p.Stop()