Pre-fetch larger areas to reduce calls to overpass; split large requests up

lib/widgets/map_view.dart

@@ -7,6 +7,7 @@ import 'package:provider/provider.dart';
 import '../app_state.dart';
 import '../services/offline_area_service.dart';
 import '../services/network_status.dart';
+import '../services/prefetch_area_service.dart';
 import '../models/osm_node.dart';
 import '../models/node_profile.dart';
 import '../models/suspected_location.dart';
@@ -194,6 +195,7 @@ class MapViewState extends State<MapView> {
     _cameraController.dispose();
     _tileManager.dispose();
     _gpsController.dispose();
+    PrefetchAreaService().dispose();
     super.dispose();
   }