Cleanup on unused code, comments, and small Javadoc updates

Cleanup on unused code, comments, and small Javadoc updates
1. Update Javadoc to specify the return object in MasterClient#addCheckpoint
2. Fix code alignment
3. Remove // TODO Auto-generated method stub
4. Fix type comment in DataServerMessage
5. Remove HdfsFileOutputStream class per discussion in the Google group
Sure, will send another PR for it