SOLR-6519: Add back "single" lock factory type to StandardDirectoryFactory (to prevent test randomization from failing). Should be discussed in separate issue.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1625724 13f79535-47bb-0310-9956-ffa450edef68
SOLR-6519: Also allow "single" for HDFS

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1625731 13f79535-47bb-0310-9956-ffa450edef68