Performance tuning: 20% improvements for simple put operations

[yjh0502]

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

[codeskyblue]

great

[sunminghong]

great

[hoisie]

This looks great, but it needs a merge with master - are you available to do it?

[yjh0502]

@hoisie I'll merge the patch with master, but not right now because I don't have a spare time right now. It may takes some time, maybe several days. I'll update this PR after merge :)