DB: add REDIS option in addition to in-memory DB

[glimchb]

example:

	// idempotent API when called with same key, should return same object
	subsys, ok := s.Nvme.Subsystems[in.NvmeSubsystem.Spec.Name]
	if ok {
		log.Printf("Already existing NvmeSubsystem with id %v", in.NvmeSubsystem.Spec.Name)
		return subsys, nil
	}

instead of s.Nvme.Subsystems map, we can use actual database like redis