Method flow

cpp/examples/broker.cpp:174–178

        tryToSend();
    }
    void flow(Sender* s, int c) {
        DOUT(std::cerr << "Queue:    " << this << "(" << name_ << ") flow: " << c << " to " << s << "\n";);
        subscriptions_[s] = c;
        tryToSend();
    }
    void subscribe(Sender* s) {
        DOUT(std::cerr << "Queue:    " << this << "(" << name_ << ") subscribe Sender: " << s << "\n";);
        subscriptions_[s] = 0;