SOLR-4413: Fix SolrCore#getIndexDir() to return the current index directory.

SOLR-4469: A new IndexWriter must be opened on SolrCore reload when the index directory has changed and the previous SolrCore's state should not be  propagated.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1447089 13f79535-47bb-0310-9956-ffa450edef68