LUCENE-6133: improve default stored fields merge algorithm

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1648332 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6142: Faster merges with Lucene41StoredFieldsFormat.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1648342 13f79535-47bb-0310-9956-ffa450edef68