private void registerCallback(MessageCallback callback) {
        ensureSetup();

        this.lock.lock();
        try {
            this.listeners.add(callback);
        } finally {
            this.lock.unlock();
        }
    }
Sends (or queues the sending of) the message encapsulated by the given message agent

@param agent the agent
helper/src/main/java/me/lucko/helper/messaging/bungee/BungeeCordImpl.java