The nation’s biggest banks and mortgage lenders have steadily amassed real estate empires, acquiring a glut of foreclosed homes that threatens to deepen the housing slump and create a further drag on the economic recovery. All told, they own more than 872,000 homes as a result of the groundswell in foreclosures, almost twice as many as when the financial crisis began in 2007, according to RealtyTrac, a real estate data provider. In addition, they are in the process of foreclosing on an additional one million homes and are poised to take possession of several million more in the years ahead.