Federal banking regulators this week took the first steps toward lowering the capital cost of holding unsecuritized residential mortgages in a bank or thrift portfolio. The huge role played by the mortgage-backed securities market – including Fannie Mae, Freddie Mac and the growing non-agency MBS business – tends to overshadow the fact that there is still a huge supply of whole loans on the books of depository institutions and other investors. As of the middle…