MAPREDUCE-5766. Moved ping messages from TaskAttempts to be at DEBUG level inside the ApplicationMaster log. Contributed by Jian He.
svn merge --ignore-ancestry -c 1572381 ../branch-2


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1572382 13f79535-47bb-0310-9956-ffa450edef68