Merge -c 1471796 from trunk to branch-2 to fix MAPREDUCE-5166. Fix ConcurrentModificationException due to insufficient synchronization on updates to task Counters. Contributed by Sandy Ryza.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1471797 13f79535-47bb-0310-9956-ffa450edef68