HADOOP-9540. Expose the InMemoryS3 and S3N FilesystemStores implementations for Unit testing. Hari

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1479985 13f79535-47bb-0310-9956-ffa450edef68
(cherry picked from commit fa56ccfd530565e33323cac7b5ebecef73703d12)