Method on_sendable

cpp/examples/broker.cpp:193–200

We have credit to send a message.

// We have credit to send a message.
void Sender::on_sendable(proton::sender &sender) {
    if (queue_) {
        auto credit = sender.credit();
        queue_->add([=]{queue_->flow(this, credit);});
    } else {
        pending_credit_ = sender.credit();
    }
}

void Sender::on_sender_close(proton::sender &sender) {
    if (queue_) {