adb: don't hold queue lock while performing callbacks.

    template <typename Fn>

    void PopAll(Fn fn) {

        std::vector<T> popped;

        {

            std::unique_lock<std::mutex> lock(mutex);

            cv.wait(lock, [this]() { return !queue.empty(); });

            popped = std::move(queue);

            queue.clear();

        }

        for (const T& t : queue) {

        for (const T& t : popped) {

            fn(t);

        }

        queue.clear();

    }

};