LUCENE-5969, LUCENE-5412: make .si immutable again, and make ancient writers read-only

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1627535 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: file mismatch detection for 5.0 fnm

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1627544 13f79535-47bb-0310-9956-ffa450edef68