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
for this PR the current conventions should be used.  Filed https://tachyon.atlassian.net/browse/TACHYON-28 for this.