[core] [node] pass thread pool impl to Map constructor

Updates mbgl::Map constructor usage everywhere

Adds NodeThreadPool implementation using AsyncQueue to call
Nan::AsyncQueueWorker from main thread

test/api/annotations.test.cpp

@@ -1,6 +1,7 @@
 #include <mbgl/test/util.hpp>
 #include <mbgl/test/stub_file_source.hpp>
 
+#include <mbgl/platform/default/thread_pool.hpp>
 #include <mbgl/annotation/annotation.hpp>
 #include <mbgl/sprite/sprite_image.hpp>
 #include <mbgl/map/map.hpp>
@@ -25,7 +26,8 @@ public:
     std::shared_ptr<HeadlessDisplay> display { std::make_shared<HeadlessDisplay>() };
     HeadlessView view { display, 1 };
     StubFileSource fileSource;
-    Map map { view, fileSource, MapMode::Still };
+    ThreadPool threadPool { 4 };
+    Map map { view, fileSource, threadPool, MapMode::Still };
 
     void checkRendering(const char * name) {
         test::checkImage(std::string("test/fixtures/annotations/") + name,