MAPREDUCE-5470. Merging change r1516623 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1516625 13f79535-47bb-0310-9956-ffa450edef68
MAPREDUCE-5476. Changed MR AM recovery code to cleanup staging-directory only after unregistering from the RM. Contributed by Jian He.
svn merge --ignore-ancestry -c 1516660 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1516662 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5124. Merge change r1516671 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1516672 13f79535-47bb-0310-9956-ffa450edef68