LUCENE-5434: NRT support for file systems that do no have delete on last close or cannot delete while referenced semantics.
SOLR-5693: Running on HDFS does work correctly with NRT search.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1565347 13f79535-47bb-0310-9956-ffa450edef68