LUCENE-5228: IndexWriter.addIndexes(Directory[]) now acquires the IW write lock on the incoming indices to ensure there are no active IndexWriters in those directories

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1561412 13f79535-47bb-0310-9956-ffa450edef68