[core] Get rid of ::unref() for Timer and AsyncTask

Not need, legacy from libuv. The RunLoop keep the main
loop running until is explicitly no longer needed.

platform/default/run_loop.cpp

@@ -13,6 +13,12 @@ namespace {
 using namespace mbgl::util;
 static ThreadLocal<RunLoop>& current = *new ThreadLocal<RunLoop>;
 
+#if UV_VERSION_MAJOR == 0 && UV_VERSION_MINOR <= 10
+void dummyCallback(uv_async_t*, int) {};
+#else
+void dummyCallback(uv_async_t*) {};
+#endif
+
 } // namespace
 
 namespace mbgl {
@@ -56,7 +62,19 @@ RunLoop* RunLoop::Get() {
 
 class RunLoop::Impl {
 public:
+    void closeHolder() {
+        uv_close(holderHandle(), [](uv_handle_t* h) {
+            delete reinterpret_cast<uv_async_t*>(h);
+        });
+    }
+
+    uv_handle_t* holderHandle() {
+        return reinterpret_cast<uv_handle_t*>(holder);
+    }
+
     uv_loop_t *loop = nullptr;
+    uv_async_t* holder = new uv_async_t;
+
     RunLoop::Type type;
     std::unique_ptr<AsyncTask> async;
 
@@ -81,6 +99,12 @@ RunLoop::RunLoop(Type type) : impl(std::make_unique<Impl>()) {
         break;
     }
 
+    // Just for holding a ref to the main loop and keep
+    // it alive as required by libuv.
+    if (uv_async_init(impl->loop, impl->holder, dummyCallback) != 0) {
+        throw std::runtime_error("Failed to initialize async.");
+    }
+
     impl->type = type;
 
     current.set(this);
@@ -90,6 +114,10 @@ RunLoop::RunLoop(Type type) : impl(std::make_unique<Impl>()) {
 RunLoop::~RunLoop() {
     current.set(nullptr);
 
+    // Close the dummy handle that we have
+    // just to keep the main loop alive.
+    impl->closeHolder();
+
     if (impl->type == Type::Default) {
         return;
     }
@@ -133,7 +161,7 @@ void RunLoop::runOnce() {
 }
 
 void RunLoop::stop() {
-    invoke([&] { impl->async->unref(); });
+    invoke([&] { uv_unref(impl->holderHandle()); });
 }
 
 void RunLoop::addWatch(int fd, Event event, std::function<void(int, Event)>&& callback) {