Redis-lru

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- Refactor RedisLRU class for a clean and neat implement.
- Stop supporting Python 2.
- Fix bugs in test cases.
- Added clean cache option at process exit.

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- Generate `unique_key` automatically so your may never get touch this argument.
- Improved exception processing, now you won't receive any exception by default.
- Added unittest for `redis_lru_cache` decorator and `RedisLRUCacheDict`
- Added more type of cache usage statistic type like `POP`, `DEL` etc.
- Added changes log.