Update RX timestamp right after assigning socket

Otherwise the socket poll callback races with the RX timeout (if
nonzero) and will immediately disconnect the socket without waiting for
it to actually finish connecting.

src/AsyncTCP.cpp

@@ -496,6 +496,7 @@ bool AsyncClient::connect(IPAddress ip, uint16_t port)
     xSemaphoreTakeRecursive(_asyncsock_mutex, (TickType_t)portMAX_DELAY);
     _conn_state = 2;
     _socket = sockfd;
+    _rx_last_packet = millis();
     xSemaphoreGiveRecursive(_asyncsock_mutex);
 
     // Socket is now connecting. Should become writable in asyncTcpSock task