Add HelloWorldThreaded and (actually) fix race

examples/HelloWorldThreaded.cpp

@@ -0,0 +1,28 @@
+#include "App.h"
+#include <thread>
+#include <algorithm>
+
+int main() {
+    /* Overly simple hello world app, using multiple threads */
+    std::vector<std::thread *> threads(std::thread::hardware_concurrency());
+
+    std::transform(threads.begin(), threads.end(), threads.begin(), [](std::thread *t) {
+        return new std::thread([]() {
+
+            uWS::App().get("/*", [](auto *res, auto *req) {
+                res->end("Hello world!");
+            }).listen(3000, [](auto *token) {
+                if (token) {
+                    std::cout << "Thread " << std::this_thread::get_id() << " listening on port " << 3000 << std::endl;
+                } else {
+                    std::cout << "Thread " << std::this_thread::get_id() << " failed to listen on port 3000" << std::endl;
+                }
+            }).run();
+
+        });
+    });
+
+    std::for_each(threads.begin(), threads.end(), [](std::thread *t) {
+        t->join();
+    });
+}