Gravar-mail: Bis(μ-3-hy­droxy­benzoato)-κ(3) O (1),O (1′):O (1);κ(3) O (1):O (1),O (1′)-bis­[(3-hy­droxy­benzoato-κ(2) O,O′)(iso­nicotinamide-κN (1))cadmium] tetra­hydrate