NewFailoverClusterClient and DB selection

[mdouchement]

Hello, I'm a little bit lost with the experimental NewFailoverClusterClient client.
It's made to connect to a Redis Sentinel but does not allow to select database like a Redis Cluster.

Since NewFailoverClusterClient returns a ClusterClient this behavior seems legit but I think we should be able to use the databases feature.

[Queetinliu]

you can put DB in redis failoveroptions.