Changed redis backend to use aioredis instead of asyncio_redis.

[hramezani]

Based on @tomchristie comment.

I have added await self._wait_for_channel(), to wait for the channel.

[JaeungJayJang]

Hello, Thank you for this great app. I see that the backend still relies on asyncio_redis, but it seems that this package is no longer maintained and aioredis is also became part of redis-py as well. I am wondering if there's any plan to replace asyncio_redis to redis-py by any chance.

Thank you!

[alex-oleshkevich]

Resolved in #111