Optimize performance and memory for HST with more than 500 to 1000 subsites (including multiple languages)

A customer needs optimal performance for 250 subsites which have about 3 translations a piece. This means, HST needs some tweaks.

Most likely, we can already gain a lot, if we make sure, that from hstConfiguration we cannot go to the Mount that belongs to it: This way, we might be able to share the same hstconfiguration model (object) for every site that points to the same hst configuration:

At this moment the only problem seems to be with

HstSiteMap#getSite() as this couples the hstconfiguration to some site instance. However, we should be able to completely remove this method, as we can and should always be able to access the current site through the Mount.