Pass opCode around, add Loop::setPostHandler

src/LoopData.h

@@ -24,6 +24,8 @@
 
 namespace uWS {
 
+struct Loop;
+
 struct LoopData {
     friend struct Loop;
 private:
@@ -31,6 +33,8 @@ private:
     int currentDeferQueue = 0;
     std::vector<std::function<void()>> deferQueues[2];
 
+    std::function<void(Loop *)> postHandler;
+
 public:
     /* Good 16k for SSL perf. */
     static const int CORK_BUFFER_SIZE = 16 * 1024;