LUCENE-5969: Move bwc blocktree writer to test module, update 50 writer to use new header reading/writing

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633465 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5969: test that default codec uses segmentheader for all files. change .si to write its ID the same way for consistency

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5969@1633471 13f79535-47bb-0310-9956-ffa450edef68