LUCENE-5969: copy over cruft for back compat

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628019 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: remove some cruft, header -> segmentHeader

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628024 13f79535-47bb-0310-9956-ffa450edef68