Make the HstRequestContext initialization done during creating instead of during the initialization valve

Originally, the HstRequestContext was primarily targeted to be used during request processing (the pipelines & valves), and not so much during the matching phase. However, for example the HstSiteMapItemHandler kicks in before the initialization valve, and thus has a not yet fully initialized HstRequestContext at that point. A simple call like RequestContextProvider.get().getContentBean() then results in errors, which is very inconvenient. For example HSTTWO-2736 is an issue resulting from this