UniversalClient to support creating FailoverClient with ReplicaOnly

[jdheyburn]

UniversalOptions doesn't support a ReplicaOnly option, but it UniversalClient can support creating FailoverClient if a MasterName is provided. a FailoverClient can support routing commands to a replica behind Sentinel if ReplicaOnly option is provided in FailoverOptions.

There is ReadOnly which is only used by cluster clients. Maybe that can be reused but it may be better to just add this to UniversalOptions.

	// Only failover clients.

	// The sentinel master name.
	MasterName string
	ReplicaOnly bool