fix #1002 ::Flush() wait for empty send buffer (#3967)

* fix #1002 ::Flush() wait for empty send buffer

* WiFiClient::Flush() guarantees that the data has been delivered
option 1 of https://github.com/esp8266/Arduino/pull/3967#discussion_r156901071
10ms max wait according to loaded tcp echo/reply scheme

libraries/Ethernet/src/EthernetUdp.h

@@ -49,6 +49,9 @@ private:
   uint16_t _remotePort; // remote port for the incoming packet whilst it's being processed
   uint16_t _offset; // offset into the packet being sent
   uint16_t _remaining; // remaining bytes of incoming packet yet to be processed
+  
+protected:
+  void clear_remaining();
 
 public:
   EthernetUDP();  // Constructor