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.cpp

@@ -118,7 +118,7 @@ size_t EthernetUDP::write(const uint8_t *buffer, size_t size)
 int EthernetUDP::parsePacket()
 {
   // discard any remaining bytes in the last packet
-  flush();
+  clear_remaining();
 
   if (recvAvailable(_sock) > 0)
   {
@@ -204,7 +204,7 @@ int EthernetUDP::peek()
   return b;
 }
 
-void EthernetUDP::flush()
+void EthernetUDP::clear_remaining()
 {
   // could this fail (loop endlessly) if _remaining > 0 and recv in read fails?
   // should only occur if recv fails after telling us the data is there, lets
@@ -216,3 +216,8 @@ void EthernetUDP::flush()
   }
 }
 
+void EthernetUDP::flush()
+{
+  endPacket();  
+}
+

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