LUCENE-5969: fix compile/javadocs, tighten up backwards codecs, add more safety to 5.x fields/vectors

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628070 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: add merge api

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1628073 13f79535-47bb-0310-9956-ffa450edef68