svn merge -c 1353807 from trunk for HDFS-3498. Support replica removal in BlockPlacementPolicy and make BlockPlacementPolicyDefault extensible for reusing code in subclasses.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1488844 13f79535-47bb-0310-9956-ffa450edef68