YARN-956. Added a testable in-memory HistoryStorage. Contributed by Mayank Bansal.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/YARN-321@1509788 13f79535-47bb-0310-9956-ffa450edef68
YARN-956. Missed new files in the previous commit.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/YARN-321@1509831 13f79535-47bb-0310-9956-ffa450edef68