Last February, I searched for a vacation rental in Door County, the wayward spear of Wisconsin that juts into Lake Michigan, for an early May getaway, confident I had beaten the summer rush to the popular vacation spot. But Airbnb occupancy calendars for lovely converted farmhouses and log cabins were already full and when I called a local vacation home rental agency, the agent said they were almost completely booked through the summer. That’s how I ended up in a one-bedroom shack last spring with a wobbly card table in an ant-infested kitchen and a new awareness of the shrinking off-season.