HDFS-4885. Improve the verifyBlockPlacement() API in BlockPlacementPolicy. Contributed by Junping Du


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1534426 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd