Fix redisKey and ZCount

[leeym]

This PR fixes two issues

• Add hash tags in redisKey: earlier in https://github.com/mennanov/limiters/pull/34 we replaced redis.Client with redis.UniversalClient. If we do use cluster client and if the cluster is large enough, it is possible that when accessing multiple keys in leaky bucket or token bucket, these keys might go different slots and it will fail with error message CROSSSLOT Keys in request don't hash to the same slot. Adding hash tags in redisKey will force them into the same slot for keys with the same prefix.
• Replace ZCount with ZCard: ZCount is O(log(N)) while ZCard is O(1). It probably doesn't matter in the most cases but it helps a little bit if the sorted set size is huge.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.05%. Comparing base (6804c15) to head (d85f4f2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #48 +/- ## ======================================= Coverage 80.05% 80.05% ======================================= Files 12 12 Lines 1449 1449 ======================================= Hits 1160 1160 Misses 212 212 Partials 77 77 ```

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[leeym]

I don't have write access to this repository so please merge the PR for me. Thanks.

[mennanov]

Great! Thanks!