LUCENE-5969: Copy block tree to backward codecs for 4.0-4.10, remove conditionals in 50 version, add getStats() TermsEnum API to remove need to expose Stats impl

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633429 13f79535-47bb-0310-9956-ffa450edef68