LUCENE-5969: move blocktree stats -> stats api

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633441 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: add missing @Override

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633442 13f79535-47bb-0310-9956-ffa450edef68