LUCENE-6123: Fix BitDocIdSet.Builder.andNot.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1647274 13f79535-47bb-0310-9956-ffa450edef68
fix IW infoStream: don't repeat 'too many merges' 4X per sec; add merge summary details about overall MB/sec

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1647279 13f79535-47bb-0310-9956-ffa450edef68