WiFiClient: apply write timeout to single chunk (#3273)

WiFiClient write timeouts introduced in #3257 applied to the whole write
operation, which could take long time if data size was large. This
change makes the timeout happen per chunk. Timeout now happens if no
data has been delivered within a given interval.

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -353,9 +353,14 @@ class ClientContext
         _written = 0;
         _op_start_time = millis();
         do {
-            _write_some();
+            if (_write_some()) {
+                _op_start_time = millis();
+            }
 
             if (!_datasource->available() || _is_timeout() || state() == CLOSED) {
+                if (_is_timeout()) {
+                    DEBUGV(":wtmo\r\n");
+                }
                 delete _datasource;
                 _datasource = nullptr;
                 break;
@@ -368,10 +373,10 @@ class ClientContext
         return _written;
     }
 
-    void _write_some()
+    bool _write_some()
     {
         if (!_datasource || !_pcb) {
-            return;
+            return false;
         }
 
         size_t left = _datasource->available();
@@ -403,7 +408,9 @@ class ClientContext
         }
         if( need_output ) {
             tcp_output(_pcb);
+            return true;
         }
+        return false;
     }
 
     void _write_some_from_cb()