LUCENE-5969: clean up unnecessary back compat and add segment header

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628679 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: clean up unnecessary back compat and add segment header

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628684 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: clean up unnecessary back compat and add segment header

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628688 13f79535-47bb-0310-9956-ffa450edef68