HDFS-6424. Merge r1599397 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1599398 13f79535-47bb-0310-9956-ffa450edef68
YARN-1913. With Fair Scheduler, cluster can logjam when all resources are consumed by AMs (Wei Yan via Sandy Ryza)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1599401 13f79535-47bb-0310-9956-ffa450edef68