Merge r1523377 from branch-2 to branch-2.1-beta for YARN-540. Race condition causing RM to potentially relaunch already unregistered AMs on RM restart (Jian He via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1523381 13f79535-47bb-0310-9956-ffa450edef68