Method on_sender_open

cpp/examples/multithreaded_client.cpp:118–124

    }

    void on_sender_open(proton::sender& s) override {
        // sender_ and work_queue_ must be set atomically
        std::lock_guard<std::mutex> l(lock_);
        sender_ = s;
        work_queue_ = &s.work_queue();
        sender_ready_.notify_all();
    }

    void on_message(proton::delivery& dlv, proton::message& msg) override {
        std::lock_guard<std::mutex> l(lock_);