Method subscribe

cpp/examples/broker.cpp:179–182

        tryToSend();
    }
    void subscribe(Sender* s) {
        DOUT(std::cerr << "Queue:    " << this << "(" << name_ << ") subscribe Sender: " << s << "\n";);
        subscriptions_[s] = 0;
    }
    void unsubscribe(Sender* s) {
        DOUT(std::cerr << "Queue:    " << this << "(" << name_ << ") unsubscribe Sender: " << s << "\n";);
        // If we're about to erase the current subscription move on