LUCENE-6131: optimize SortingMergePolicy

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1647588 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6120: Fix MockDirectoryWrapper close() handling

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1647600 13f79535-47bb-0310-9956-ffa450edef68