[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.
Will try my best to see whether it is possible and not too tricky to keep it out of Block Manager.