Merge r1516333 from branch-2 to branch-2.1-beta for YARN-881. Priority#compareTo method seems to be wrong. (Jian He via bikas)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1516342 13f79535-47bb-0310-9956-ffa450edef68