svn merge -c 1503720 from trunk for HDFS-4992. Make balancer's mover thread count and dispatcher thread count configurable.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1503721 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1503744 to revert MAPREDUCE-5317.  Stale files left behind for failed jobs


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1503747 13f79535-47bb-0310-9956-ffa450edef68