MBPlacemark should store property values parsed from GeoJSON

Each property that’s parsed out of MBPlacemark’s backing GeoJSON is potentially expensive, especially when accessed frequently. It’s easy to come up with a pathological case that involves putting addressDictionary fields into a table view cell. Since the backing GeoJSON is immutable, these property values should be lazily computed then stored for the lifetime of the object.

定位 MBPlacemark，并检查其底层 GeoJSON 如何提供 addressDictionary 等属性。确认哪些值会在重复访问时重新计算，然后添加生命周期缓存，同时保留现有的属性行为。当重复访问属性不再重新解析不可变的 GeoJSON 时，即表示完成。