YARN-956. Added a testable in-memory HistoryStorage. Contributed by Mayank Bansal.
svn merge --ignore-ancestry -c 1509788 ../../branches/YARN-321-old/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/YARN-321@1556721 13f79535-47bb-0310-9956-ffa450edef68