By The Celebrities Psychic NANCY BRADLEY Disembodied voices in an empty room. Glasses clinking when no one is there. A few years ago, David Mintz, then owner/manager of Rivendell Books at 352 Main Street, Placerville, California, heart of the gold rush country, made an astonishing find. His store was haunted. What started as an ordinary day would prove fateful. This particular cloudy winter morning he stepped from his car as he always did, and walked the few steps to the front door of his business. It was there he crossed the threshold to the unknown. As he fumbled to put the key in the lock to open the store for the day, he heard voices. Startled, he carefully and silently removed the key