[core] restructure TileSource construction and callbacks

src/mbgl/tile/vector_tile_source.hpp

@@ -7,17 +7,17 @@
 namespace mbgl {
 
 class FileSource;
+class AsyncRequest;
 
 class VectorTileSource : public GeometryTileSource {
 public:
-    VectorTileSource(const Resource&, FileSource&);
+    VectorTileSource(GeometryTileData&, const Resource&, FileSource&);
     virtual ~VectorTileSource() = default;
 
-    std::unique_ptr<AsyncRequest> monitorTile(const Callback&) override;
-
 private:
     const Resource resource;
     FileSource& fileSource;
+    std::unique_ptr<AsyncRequest> request;
 };
 
 } // namespace mbgl