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/trunk@1523376 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd