LUCENE-4952: add method to force DrillSideways to keep all sub-scorers on the doc being collected

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1471735 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-4951: cutover another freq -> cost

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1471741 13f79535-47bb-0310-9956-ffa450edef68