private State state = State.Running;
    private final IncomingQueue incomingQueue = new IncomingQueue(lock);
    private final OutgoingQueue outgoingQueue = new OutgoingQueue(incomingQueue, lock);
    private final ConnectionSet connections = new ConnectionSet(incomingQueue, outgoingQueue);

    /**
     * @param errorHandler Notified when some async activity fails. Must be thread-safe.
     */