Thousands of mall-based stores are shutting down in what’s fast becoming one of the biggest waves of retail closures in decades.More than 3,500 stores are expected to close in the next couple of months.Department stores like JCPenney, Macy’s, Sears, and Kmart are among the companies shutting down stores, along with middle-of-the-mall chains like Crocs, BCBG, Abercrombie & Fitch, and Guess.Some retailers are exiting the brick-and-mortar business altogether and trying to shift to an all-online model.