XWIKI-8911: Upgrade to Apache Lucene and Solr 4.6.1
* Starting with Solr 4.4 the cores are discovered automatically if we follow the recommended directory structure inside solr data folder.
* I didn't upgrade Restlet, but from my tests, Solr 4.6.1 works fine with the (older) version of Restlet that is used by our Rest module.