Poly[diaquabis(μ 3 -1 H -benzimidazole-5,6-dicarboxylato-κ 4N 3 : O 5 , O 5′ : O 6 )bis(μ 2 -1 H ,3 H -benzimidazolium-5,6-dicarboxylato-κ 3O 5 , O 5′ : O