use Atomic value to improve performance

[siddontang]

Now we use RwLock<f64>, but we can use Atomic U64 like golang client to improve the performance.

But be careful that now the stable rust only supports AtomicUsize, so we must use this in 64 bit system only.

And we should do benchmark to see whether this can improve the performance or not.

[overvenus]

Just a quick reminder.

• Unstable Struct std::sync::atomic::AtomicU64
• Add more integer atomic types
• rust-rfc: Add more integer atomic types
• rust-rfc: Add a generic Atomic type lots of interesting discussion.
• atomic-rs, an interesting crate, atomic anything implements Copy.