Support seamless HST framework support for asynchronous loading of hst components

Since we support componentrendering, it is now a small step to have complete framework support for asynchronous component rendering : This means, that a developer can mark a hst component to be loaded asynchronous, which means that the HST will, most likely in its aggregation valve, will include a hst componentrendering link, and does an ajax call to fetch the asynchronous loaded hst component

This is very useful combined with the pagecache valve: A hst component will be possible to be marked as uncacheable. In that case, the page might still be cached if the hst component is also marked to be loaded asynchronous

Thanks Frank for the very good ideas and inputs on asynchronous loading. This is again a killer feature. Combined with page cache valve this is going to be very nice....