In the vegetable-calm existence of a chain grocery store a housewife's dissatisfaction with her cantaloupes may be considered a major misfortune. Last week in Cleveland organized Labor gave the Great Atlantic & Pacific Tea Co.'s 300 stores a taste of real trouble. Unions began picketing the company's warehouses. Trucks hauling merchandise from warehouse to store were halted. "Flying squadrons" rolled from one red-fronted store to another, growling "close up or we'll come back and close you." A few plate-glass windows got cracked. Meat Cutters', Bakers' and Retail Clerks' & Managers'...