In schema rename "partitioned" handler to "drupal" for consistency with 7.x

In solrconfig.xml, we have been using a handler named partitioned, for no reason other than it was in the default solrconfig.xml. For consistency with 7.x, let's rename it to "drupal" instead. Not that we never use an explicit wt= param for this since it's the default, so this should not cause any breakage.