count-index.go
func NewExpiringCountIndex(resolution Meters, expiration Minutes) *CountIndex {
	newExpiringCounter := func() interface{} {
		return newExpiringCounter(expiration)
	}

	return &CountIndex{newGeoIndex(resolution, newExpiringCounter), make(map[string]Point)}
}
func NewExpiringCountIndex(resolution Meters, expiration Minutes) *CountIndex {
	newExpiringCounter := func() interface{} {
		return newExpiringCounter(expiration)
	}

	return &CountIndex{newGeoIndex(resolution, newExpiringCounter), make(map[string]Point)}
}
// NewExpiringCountIndex creates an index, which maintains an expiring counter for each cell.