[node] Destroy the RunLoop with the last Map object

The last map object to get destroyed will also destroy
the RunLoop (which will also destroy the AsyncTask).

This will clear any reference to the Node default main
loop and let it close.

Downside is that we need to make sure we are always calling
`map.release()`.

platform/node/src/node_map.hpp

@@ -3,6 +3,7 @@
 #include <mbgl/map/map.hpp>
 #include <mbgl/storage/file_source.hpp>
 #include <mbgl/platform/default/headless_view.hpp>
+#include <mbgl/util/run_loop.hpp>
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-parameter"
@@ -45,6 +46,7 @@ public:
     std::unique_ptr<mbgl::FileRequest> request(const mbgl::Resource&, Callback);
 
     mbgl::HeadlessView view;
+    std::shared_ptr<mbgl::util::RunLoop> loop;
     std::unique_ptr<mbgl::Map> map;
 
     std::exception_ptr error;