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
HADOOP-9935. Merging change r1523403 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1523404 13f79535-47bb-0310-9956-ffa450edef68