Real estate brokerage executives are increasingly confident about housing markets and the economy, but cite recruiting and training the next generation of agents as top concerns.    That's according to a "Thought Leader" survey of more than 850 brokerage executives at leading franchises and independent brokerage firms that handled more than one-third of U.S. residential real estate transactions last year.The survey, conducted in September by real estate marketing technology firm Imprev Inc., found 81.5 percent of brokerage executives were more confident about the housing market than they were in January.Most (70.8 percent) said they were "highly confident" (6.2 percent) or "confident" (64.6 percent) that the housing market would continue to improve in the ...