[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_mapbox_gl_native.cpp

@@ -12,19 +12,21 @@
 
 namespace node_mbgl {
 
-mbgl::util::RunLoop& NodeRunLoop() {
-    static mbgl::util::RunLoop nodeRunLoop;
-    return nodeRunLoop;
+std::shared_ptr<mbgl::util::RunLoop> NodeRunLoop() {
+    static std::weak_ptr<mbgl::util::RunLoop> nodeRunLoop;
+
+    auto loop = nodeRunLoop.lock();
+    if (!loop) {
+        loop = std::make_shared<mbgl::util::RunLoop>();
+        nodeRunLoop = loop;
+    }
+
+    return std::move(loop);
 }
 
 }
 
 NAN_MODULE_INIT(RegisterModule) {
-    // This has the effect of:
-    //   a) Ensuring that the static local variable is initialized before any thread contention.
-    //   b) unreffing an async handle, which otherwise would keep the default loop running.
-    node_mbgl::NodeRunLoop().stop();
-
     node_mbgl::NodeMap::Init(target);
     node_mbgl::NodeRequest::Init(target);