Add support for redis sentinel config by using redis.UniversalClient which auto-switches based on config

[andsens]

This is one of the nice code changes where we can add more capability with the tiniest diff:
redis-go has a UniversalClient which automatically detects which redis client to use (simple, cluster, or sentinel) based on this piece of code.

func NewUniversalClient(opts *UniversalOptions) UniversalClient {
	if opts.MasterName != "" {
		return NewFailoverClient(opts.failover())
	} else if len(opts.Addrs) > 1 {
		return NewClusterClient(opts.cluster())
	}
	return NewClient(opts.simple())
}

Meaning, this PR introduces support for redis sentinel (HA failover) and also obsoletes the special redis_cluster_options setting. I haven't removed it though (legacy support). At some point a deprecation warning might be in order though.

[andsens]

Woops. UniversalOptions only accepts Addrs, not Addr like with the SimpleOptions. I'll update the PR

[andsens]

Hmpf, I don't see a way of doing this cleanly while preserving legacy configs. I suggest breaking compatibility and simplifying the entire thing (looks around, ducks).