svn merge -c 1523400 from trunk for HDFS-5188. In BlockPlacementPolicy, reduce the number of chooseTarget(..) methods; replace HashMap with Map in parameter declarations and cleanup some related code.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1523401 13f79535-47bb-0310-9956-ffa450edef68