Performance tuning: 20% improvements for simple put operations

I tuned some function in critical path.
 - Change fmt.Sprintf to bytes.Buffer
 - Change string operations to byte operations
 - Change default buffer size to handle small-sized request better

These update shows 20% improvement for simple set operations. See following link for detail: https://docs.google.com/spreadsheet/ccc?key=0AjFZD5XgU6lFdEhPS21Pd0lpYjFKcGJLalRCdG9UZ3c#gid=0