YARN-925. Defined a Reader Interface for HistoryStorage. Contributed by Mayank Bansal.
svn merge --ignore-ancestry -c 1508228 ../../branches/YARN-321-old/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/YARN-321@1556719 13f79535-47bb-0310-9956-ffa450edef68