Support Hst Model invalidation, rebuilding and concurrency

Note the invalidation has two parts

1) invalidation via jcr listeners (async)
2) instant invalidation which cannot wait for an async jcr event (for example channel mngr modifications : An editor needs to see his changes instantly on the next request and it is not
allowed that during the next request the old model is used because the async jcr event did not yet arrive)

Also we very carefully have to sort out potential deadlock issues since channels are currently being fetched over java calls instead of http servlet requests