[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.cpp

@@ -439,6 +439,7 @@ void NodeMap::release() {
     });
 
     map.reset(nullptr);
+    loop.reset();
 }
 
 NAN_METHOD(NodeMap::DumpDebugLogs) {
@@ -458,6 +459,7 @@ NodeMap::NodeMap(v8::Local<v8::Object> options) :
         Nan::HandleScope scope;
         return Nan::Has(options, Nan::New("ratio").ToLocalChecked()).FromJust() ? Nan::Get(options, Nan::New("ratio").ToLocalChecked()).ToLocalChecked()->NumberValue() : 1.0;
     }()),
+    loop(NodeRunLoop()),
     map(std::make_unique<mbgl::Map>(view, *this, mbgl::MapMode::Still)),
     async(new uv_async_t) {
 
@@ -484,7 +486,7 @@ std::unique_ptr<mbgl::FileRequest> NodeMap::request(const mbgl::Resource& resour
 
     // This function can be called from any thread. Make sure we're executing the
     // JS implementation in the node event loop.
-    req->workRequest = NodeRunLoop().invokeWithCallback([this] (mbgl::Resource res, Callback cb2) {
+    req->workRequest = loop->invokeWithCallback([this] (mbgl::Resource res, Callback cb2) {
         Nan::HandleScope scope;
 
         auto requestHandle = NodeRequest::Create(res, cb2)->ToObject();