CoreSorter is partially broken, thus core loading order is sub-optimal

In SOLR-7280, it Solr was supposedly improved to load cores in a more optimal order in SolrCloud, considering the state of collections across the cluster.  The CoreContainer uses CoreSorter for this.  Unfortunately, CoreSorter's attempts to gather statistics early on in CoreSorter.init() operate on an empty list and do nothing (i.e. dead code). The root cause is that getCloudDescriptors() works by looking at cc.getCores however at this stage there are no cores ! (We haven't sorted them so they certainly haven't been loaded yet.   I have a fix.

This might be classified as a bug fix but the core load order is more about optimization, so a fix to a wrong/incomplete optimization isn't really a bug from a user's point of view.