LUCENE-6015: DocIdSetBuilder: Use the sparse set until more than 1/16 of the longs have one bit set.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1633305 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6015: Fix TestDocIdSetBuilder.testDense expectations.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1633309 13f79535-47bb-0310-9956-ffa450edef68