HADOOP-9173. Merging changes r1428970 and r1428972 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1429030 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1429040 FIXES: MAPREDUCE-4832. MR AM can get in a split brain situation. Contributed by Jason Lowe


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1429046 13f79535-47bb-0310-9956-ffa450edef68