SOLR-4640: CachingDirectoryFactory can fail to close directories in some race conditions.
SOLR-4637: Replication can sometimes wait until shutdown or core unload until removing some tmp directories.
SOLR-4638: DefaultSolrCoreState#getIndexWriter(null) is a way to avoid creating the IndexWriter earlier than necessary, but it's not implemented quite right.
SOLR-4597: fixes, improvements
SOLR-4629: Stronger testing.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_2@1460515 13f79535-47bb-0310-9956-ffa450edef68