Memory leak when using preferredSitemapItem linking in combination with very very many hst configuration changes

The DefaultHstLinkCreator has a map for caching sub-locationmaptree's

This map can better be flushed when the HST model gets reloaded.

Although it is only used when injecting preferred sitemap items in a hst:link, and it will hardly ever be used, if it is used, it is a very tiny possible leak.