YARN-744. Race condition in ApplicationMasterService.allocate .. It might process same allocate request twice resulting in additional containers getting allocated. (Omkar Vinit Joshi via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1543707 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch: upstream/trunk