Merge -c 1489187 from trunk to branch-2 to fix MAPREDUCE-5220. Fix backward incompatibility for TaskCompletionEvent by adding back setTaskID. Contributed by Zhijie Shen.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1489188 13f79535-47bb-0310-9956-ffa450edef68