Combined experiment performance improvements

[gnanou]

Reduce the number of redundant Redis operations executed for the same request. This way the performance of a test with combined experiments is expected to improve.

• Cache Experiment#has_winner? result for the lifetime of an experiment instance
• Remove redundant calls to Redis in Experiment#save
• Cleanup Split user old experiments only once during the lifetime of a User instance
• Specs added

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Resolves issue #573

[andrehjr]

Thanks so much for your PR, I made a lot of tests and the number of calls to redis reduced a lot.

I see that there's still a lot to be on how Split handles calls to redis!

[gnanou]

That's great, thanks a lot for reviewing!
Btw, have you thought yet when the next release will be? Just wondering, I will be watching it in any case :)