RedIStack doesn't vendor concurrency APIs

[Joannis]

Concurrency is increasingly a critical part of the Swift ecosystem, even more so on Server than other Swift ecosystems. RediStack doesn't currently support concurrency at all, and users are manually .get()-ing EventLoopFutures. This is not a very ergonomic/modern API, and there are many more (potential) improvements beyond a simple await we could do. Like exposing an AsyncSequence for pub/sub.