HDFS-5868. Make hsync implementation pluggable. (Contributed by Buddy Taylor)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1569978 13f79535-47bb-0310-9956-ffa450edef68
YARN-713. Fixed ResourceManager to not crash while building tokens when DNS issues happen transmittently. Contributed by Jian He.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1569979 13f79535-47bb-0310-9956-ffa450edef68