YARN-956. Added a testable in-memory HistoryStorage. Contributed by Mayank Bansal.
svn merge --ignore-ancestry -c 1556721 ../YARN-321


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1562178 13f79535-47bb-0310-9956-ffa450edef68
YARN-956. Missed new files in the previous commit.
svn merge --ignore-ancestry -c 1556722 ../YARN-321


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1562179 13f79535-47bb-0310-9956-ffa450edef68