Provide externally-managed Redis client instance

[Codesleuth]

I need to manage the Redis server connection myself. Currently I see the server dying when Redis does not connect, which can only be prevented with listening to redis.on("error", (err) => { ... }). I wish to manage this myself and so passing in a Redis client would be beneficial.

I've checked through the code and can't see any conflicting scenarios with this so I will PR in my proposal in line with my needs for now.