SOLR-5986: Add an ExitableDirectoryReader in Lucene and use that to support exiting of long running queries in Solr.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1627622 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6564: Fix failing ExitableDirectoryReader test in Solr

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1627678 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: add remaining infos/deletions safety

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1627805 13f79535-47bb-0310-9956-ffa450edef68
merge trunk

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1627806 13f79535-47bb-0310-9956-ffa450edef68