Move to use redis instead of coredis

[hxyannay]

Hey.
I was reading the docs and saw that the library uses coredis for the async usage of redis.
I assume that this is due to lacking support for asyncio in redis when it was developed.

However, the official library for redis is much more maintained and is the more logical library to use, and by now it has full support for asyncio.

Are you open to the idea of moving to use the official library instead of coredis? If so I can try to open a PR myself for that.

Thanks!

[alisaifee]

I'm open to the idea if it can be done in a backward compatible way

[tuukkamustonen]

Would it be "just" re-writing https://github.com/alisaifee/limits/blob/master/limits/aio/storage/redis.py using redis? Do you foresee some issues there, @alisaifee?