SOLR-5907: The hdfs write cache can cause a reader to see a corrupted state. It now defaults to off, and if you were using solr.hdfs.blockcache.write.enabled explicitly, you should set it to false.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene_solr_4_7@1581176 13f79535-47bb-0310-9956-ffa450edef68