implement async replies in mDNS library

No need to call mdns.update() from loop() any more.

libraries/ESP8266WebServer/examples/HelloServer/HelloServer.ino

@@ -69,6 +69,5 @@ void setup(void){
 }
  
 void loop(void){
-  mdns.update();
   server.handleClient();
 } 

libraries/ESP8266WebServer/examples/SDWebServer/SDWebServer.ino

@@ -156,6 +156,5 @@ void setup(void){
 }
  
 void loop(void){
-  mdns.update();
   server.handleClient();
 }