Merge -c 1477897 from branch-1 to branch-1.2 to fix MAPREDUCE-5198. Fix a race condition during TaskTracker re-init which was causing failures since task directories were being cleaned up in multiple threads. Contributed by Arpit Gupta.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.2@1477898 13f79535-47bb-0310-9956-ffa450edef68