Method on_receiver_open

cpp/src/credit_test.cpp:172–178

    }

    void on_receiver_open(proton::receiver &r) override {
        receiver_ = r;
        next_idle_ = proton::make_work(&tester::first_idle, this);
        proton::work call_on_server_idle(make_work(&tester::on_server_idle, this));
        srv_->notify_on_idle(r.work_queue(), call_on_server_idle);
        r.add_credit(initial_credit_);
    }

    void on_message(proton::delivery &d, proton::message &m) override {
        received_++;