make timestamp() function cross-platform and thread-safe

Created by: ulyachka

fix for https://github.com/OpenXRay/xray-16/commit/e6402cdefe35b0b852605d18c1af11a58f973460 and https://github.com/OpenXRay/xray-16/commit/42ef1ccc58345f4935ee0d3e6de4f5c504f155eb