Investigate if ioredis can be used as client

[dlebech]

Would it be possible to remove the client restriction here and either:

1. Allow everything, thereby putting responsibility on the library user (This would allow mock objects)
2. Add specific support for ioredis

I'm leaning towards 1 and then explicit stating in the documentation that redis is officially supported but other libraries can (possibly) be used to.

[stefanosala]

I'm for 1. too, but we could at least check if the methods we need are in the passed client object and if they return promises.

[jeffmcaffer]

#1 would work for me as well.

[dlebech]

@stefanosala @jeffmcaffer I've now tested the current suite with ioredis and there are 8 failing tests. It's possible it's something minor, but at least it's not just a matter of removing the client restriction without any further changes. I will leave this open for now, but I don't think I will have time in the near-future to deep dive into this and make it work for both clients.

However, I will still remove the client restriction so a mock object can be passed and leave responsibilty to the developer :-) This change is coming up soon.