While employees for Suffolk Downs lamented the end of horse racing and loss of jobs there with the track’s failed casino bid, a development expert said the prime 160-acre property is a new, much-needed Seaport District waiting to happen.David Begelfer, CEO of the real estate development group NAIOP Massachusetts, said Suffolk is a “hot” property due to its sheer size and its proximity to downtown Boston, Logan International Airport and heavily traveled thoroughfares.