Investiga lazy loading of hstconfiguration

What we can improve is the following:

1) Only actively load the hosts, mounts, hst:site and hst:sitemaps : These are needed for linkrewriting including cross domain linkrewriting (although, possibly, the hst:sitemaps (and sitemapitemhandlers) could also be lazily loaded (when there is a cross domain link) just like the hstconfiguration)
2) After a Mount or SitemapItem has been resolved, load its HstConfiguration : OPTIONALLY further improvement: Only load (+enhance) the root component configuration that the sitemap item points to: so even within a single hstconfig, do lazy loading!!)

For this scenario to work well, I think we need to attach the SitemapItemHandlerRegistry and HstComponentRegistry to a Mount: thus also create two registries per mount. The current solution in the hst with registries might give race conditions already