Support async command evaluation

[mberndt123]

Resolves #916

Hey @mijicd,

I've hacked together something to implement the improved async support that I laid out in #916. The code could certainly be made a little bit prettier here and there, but the primary aim here is to try out the idea and see if you consider this a viable way forward.

The essential changes are:

• change from requests.offer(…stuff…) *> promise.await to requests.offer(…stuff…).as(promise.await)), which makes it possible to continue processing as soon as the request was placed in the queue (SingleNodeExecutor.scala:35)
• add a type parameter to the Redis trait to indicate whether it will return UIO[IO[RedisError, A]] or IO[RedisError, A] (Redis.scala:23)
• change Redis.singleNode and Redis.local to provide both a synchronous and an asynchronous implementation of the Redis client (Redis.scala:45)

The resources like Queues and the Redis connection are shared between the synchronous and asynchronous Redis client, see the changes in SingleNodeExecutor.

The biggest drawback right now is that I haven't yet looked into the cluster support, so the cluster ZLayer doesn't give you an AsyncRedis object. I'd like to get some feedback on the general approach first before tackling that.

[CLAassistant]

All committers have signed the CLA.

[mijicd]

A few questions/nitpicks, otherwise LGTM