Add support for service discovery.
The newly-released Service Discovery framework gives us the interesting opportunity to make RediStack aware of complex service discovery tools. This patch supplies a simple adaptor to integrat Service Discovery with RediStack's pooled client, allowing users to work with arbitrary service discovery systems.
Right now this patch is under-tested because I wanted to get some confidence that you're happy with the approach before I go too far. Additionally, it's plausible that you might have some concerns about the .exact dependency on service discovery. If that's true we'll need to see if we can encourage the service discovery folks to tag a 1.0 before we merge this.