Better support for checking site availability

This is a request to have the possibility to let load balancers known when the HST model is loaded, so that user requests can start being directed to the site.

In a project on Hippo 10, soon going to 11, with a very large HST model, the HstPingServlet is used by load balancers to check the site availability. HstPingServlet checks for repository read access but is (by default) excluded from the HstFilter so returns OK before the model is present. This was not wanted (after startup, first user request takes > 1 min) therefore the exclusion was undone.

Including /ping in the HstFilter processing can lead to HST session pool exhaustion and incorrect Lucene index (double search results) so is not ideal either.

Perhaps we can add a check in the HstPingServlet for the availability of the HST model, and if not available, initiate the loading but not wait for it? Any way it still should be returning efficiently and quickly.