HDFS-4259. Improve pipeline DN replacement failure message. Contributed by Harsh J. (harsh)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1439127 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9241. DU refresh interval is not configurable. Contributed by Harsh J. (harsh)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1439130 13f79535-47bb-0310-9956-ffa450edef68