Custom Client Factory

[mjarmoc]

This PR introduces the abiilty to provide a custom instance of cache client. The intended purpose is to wrap IORedis instance in your custom instance, for example not to expose all public methods, override the default or provide custom ones.

The most radical use case would be also to replace IORedis with another Redis lib (but at some level of compatibility), so it makes the whole library, not vendor locked in the future.

On the type/semantic level, it changed the naming of the Clients Pool from misguiding RedisClient to RedisClients.

[mjarmoc]

@skunight could you take a look at it :)?