**Describe the solution you'd like**

We already can run clickhouse-server without configured ZooKeeper (the tables will start in read-only mode)
and with configured ZooKeeper, if it starts to be inaccessible at runtime (the tables will be read-only until the session is re-established).

Let's also make it possible to start up when ZooKeeper is unavailable.

**Additional context**

If that will be very hard to implement, we can consider other options:
- lazy attaching of the tables after sever start-up;
- a maintenance mode when the tables are not attached at start-up and require manual ATTACH queries.
{
    std::lock_guard lock(last_queue_update_exception_lock);
    return last_queue_update_exception;
}
void StorageReplicatedMergeTree::queueUpdatingTask()
{
    if (!queue_update_in_progress)
    {
        last_queue_update_start_time.store(time(nullptr));
        queue_update_in_progress = true;
    }
    try
    {
        queue.pullLogsToQueue(getZooKeeperAndAssertNotReadonly(), queue_updating_task->getWatchCallback(), ReplicatedMergeTreeQueue::UPDATE);
        last_queue_update_finish_time.store(time(nullptr));
        queue_update_in_progress = false;
    }
    catch (const Coordination::Exception & e)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
        std::lock_guard lock(last_queue_update_exception_lock);
        last_queue_update_exception = getCurrentExceptionMessage(false);
        if (e.code == Coordination::Error::ZSESSIONEXPIRED)
        {
            restarting_thread.wakeup();
            return;
        }