Cleanup on code and add small Javadoc updates

Cleanup on code and add 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 spacing in swith-cases statements in MasterInfo class
5. Fix type comment in DataServerMessage
Sorry @haoyuan, so should I change the code-formatter or do you prefer to have the case in line with switch ?