Make it possible to collapse results under jcr:content nodes

In some use cases a lot of search results live in descendants of jcr:content nodes although the caller is only interested in those jcr:content nodes as results (that post filtering being applied using primary types or other constraints) so that the Solr query could be optimized by applying Solr's Collapse query parser in order to collapse such documents and therefore allowing huge improvements in performance, especially on the query engine not having to filter e.g. 90% of the nodes returned by the index.