ISLESBORO, Maine — In small Maine towns, the local general store sells sundries, sure, but also doubles as a gathering place and one of the centers of the community. That’s even more true on the state’s offshore islands, where stores help those who live in remote locations survive the long winters — or just get through the mornings when they run out of milk and coffee. Islesboro, a long, narrow island in Penobscot Bay, 3 miles east of Lincolnville Beach, has two stores — the Island Market and Durkee’s General Store, on the north side.