[node] switch to NodeRequest member fn callback

For (hopefully) better performance than creating a new v8::Context to
wrap each callback while still avoiding leaking memory with v8::FunctionTemplate.

Adds a JavaScript shim in front of module.exports.Map to wrap the
req.respond API internally and preserve the public callback-passing
API, while still exporting the correct prototype.

platform/node/src/node_map.hpp

@@ -23,7 +23,7 @@ public:
 
     static Nan::Persistent<v8::Function> constructor;
 
-    static NAN_MODULE_INIT(Init);
+    static void Init(v8::Local<v8::Object> exports);
 
     static void New(const Nan::FunctionCallbackInfo<v8::Value>&);
     static void Load(const Nan::FunctionCallbackInfo<v8::Value>&);