[SPARK-2288] Hide ShuffleBlockManager behind ShuffleManager

By Hiding the shuffleblockmanager behind Shufflemanager, we decouple the shuffle data's block mapping management work from Diskblockmananger. This give a more clear interface and more easy for other shuffle manager to implement their own block management logic. the jira ticket have more details.
can u add a todo here that getValues should bypass getBytes to use stream based APIs? Otherwise this uses a lot of memory during external sort merge.