rawkv: add TTL argument for client method CompareAndSwap

[tutububug]

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[tutububug]

to fix issue #371 .

[disksing]

Hi, @tutububug , can you make it an option so that it won't break the API?

[tutububug]

Actually, in our use scenario, we use the CAS method to implement the distribute lock function.
The key names 'lock', and the value names 'locker'. Users can unlock a key only when its value equals the value the user passes. In normal cases, there is no problem, but when the locker client crashes, there should be a semantic 'lock timeout', so the TTL here needs to be used and added to the CAS parameters.

[Smityz]

@disksing how about adding a new function CompareAndSwapTTL