HDFS-5809. BlockPoolSliceScanner and high speed hdfs appending make datanode to drop into infinite loop (cmccabe)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1610791 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1610829 FIXES: Addendum patch for HADOOP-10468 TestMetricsSystemImpl.testMultiThreadedPublish fails intermediately. Contributed by Akira AJISAKA


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1610830 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6456. Merging change r1610840 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1610843 13f79535-47bb-0310-9956-ffa450edef68