Make CoreSorter class configurable

I would like to be able to configure how cores are sorted before being loaded to accommodate more specific needs.

For this purpose, I propose to make the CoreSorter instance configurable, pretty much like other services (e.g., ConfigSetService, CoresLocator) are already configurable. Attached PR creates a new "coreSorter" parameter in solr.xml in order to achieve this.

Example usage:

<solr>
  <str name="coreSorter">com.myorg.CustomCoreSorter</str>
</solr>