YARN-934. Defined a Writer Interface for HistoryStorage. Contributed by Zhijie Shen.
svn merge --ignore-ancestry -c 1556711 ../YARN-321


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1562175 13f79535-47bb-0310-9956-ffa450edef68
YARN-925. Defined a Reader Interface for HistoryStorage. Contributed by Mayank Bansal.
svn merge --ignore-ancestry -c 1556719 ../YARN-321


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1562176 13f79535-47bb-0310-9956-ffa450edef68