LUCENE-5689: FieldInfo.setDocValuesGen should not be public

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1596553 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5618, LUCENE-5636: write each DocValues update in a separate file; stop referencing old fieldInfos files

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1596570 13f79535-47bb-0310-9956-ffa450edef68