LUCENE-5992: encode version using 3 ints, not String, for Lucene 5.x indices; add SI tests; remove FieldInfos from SegmentInfosWrite.write

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1629775 13f79535-47bb-0310-9956-ffa450edef68