ESP8266WebServer - Drop inactive connection when another is waiting to improve page load time (#8216)

* ESP8266WebServer - drop current HC_WAIT_READ connection sooner when another has data

Safari sometimes opens two connections when loading a page and only
sends a request over the second one, resulting in a 5 second wait
(HTTP_MAX_DATA_WAIT) before the request is processed. This commit
drops the current connection after 30ms (HTTP_MAX_DATA_AVAILABLE_WAIT)
when there is a new connection with data available or the buffer of
pending TCP clients is full (currently 5).

doc/esp8266wifi/server-class.rst

@@ -49,6 +49,8 @@ Other Function Calls
 .. code:: cpp
 
     bool  hasClient () 
+    size_t  hasClientData () 
+    bool  hasMaxPendingClients ()
     bool  getNoDelay () 
     virtual size_t  write (const uint8_t *buf, size_t size) 
     uint8_t  status ()