Doug Tompkins, prematurely deceased at age 72 by way of a kayaking accident, leaves a cadre of older conservationists behind, many of whom are still on the front lines of nature protection. Still, his passing sounded the note of an era’s end and a changing of the guard. Old time enviros turned out in good form for Tompkins’ memorial service last week, held at Herbst Pavillion in San Francisco’s Fort Mason. Herbst is usually filled to the rafters with antiques, art work, used