Improve efficiency with periodic location checks

[bkeepers]

By default, CoreLocation updates location every second or so, which is a much higher frequency than what is needed for this use case. This unnecessarily drains the battery. It would probably be acceptable to only check location every 30-120 seconds, as long as those location checks were high accuracy. But, it is not easy to get updates with a high accuracy at a low frequency from CoreLocation.

This thread has some ideas about how to do it:
https://stackoverflow.com/questions/19042894/periodic-ios-background-location-updates/24666487