**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.
{
    if (fetcher.blocker.isCancelled())
    {
        disable_reason = fmt::format("Not executing fetch of part {} because replicated fetches are cancelled now.", entry.new_part_name);
        return false;
    }
    size_t busy_threads_in_pool = CurrentMetrics::values[CurrentMetrics::BackgroundFetchesPoolTask].load(std::memory_order_relaxed);
    if (busy_threads_in_pool >= replicated_fetches_pool_size)
    {
        disable_reason = fmt::format("Not executing fetch of part {} because {} fetches already executing, max {}.", entry.new_part_name, busy_threads_in_pool, replicated_fetches_pool_size);
        return false;
    }
    if (replicated_fetches_throttler->isThrottling())
    {
        disable_reason = fmt::format("Not executing fetch of part {} because fetches have already throttled by network settings "
                                     "<max_replicated_fetches_network_bandwidth> or <max_replicated_fetches_network_bandwidth_for_server>.", entry.new_part_name);
        return false;
    }
    return true;
}
bool StorageReplicatedMergeTree::partIsAssignedToBackgroundOperation(const DataPartPtr & part) const
{
    return queue.isVirtualPart(part);
}
void StorageReplicatedMergeTree::mergeSelectingTask()
{
    if (!is_leader)
        return;
    const auto storage_settings_ptr = getSettings();
    const bool deduplicate = false;