merge YARN-365 from trunk. Change NM heartbeat handling to not generate a scheduler event on each heartbeat. Contributed by Xuan Gong.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1450008 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd