SOLR-5986: Add an ExitableDirectoryReader in Lucene and use that to support exiting of long running queries in Solr. (Merge from trunk r1627622)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1627635 13f79535-47bb-0310-9956-ffa450edef68