Guard the HstFreeMarkerServlet initialization against heavy initial concurrent usage

The HstFreeMarkerServlet provides a TagLibFactory instance which is saved and shared via the ServletContext between all requests.
As this TagLibFactory is currently initialized during the first request, this can lead to race conditions if the servlet is hit by heavy concurrent initial usage (e.g. like with a benchmark).

The TagLibFactory initialization however can just as well be done during the init phase of the servlet itself, which is synchronized by the container itself and thus no longer be subject to race conditions.