**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.
{
    String full_zk_path = getZooKeeperName() + getZooKeeperPath();
    if (!restorer.getRestoreCoordination()->acquireInsertingDataIntoReplicatedTable(full_zk_path))
    {
        
        
        return;
    }
    if (!restorer.isNonEmptyTableAllowed())
    {
        bool empty = !getTotalActiveSizeInBytes();
        if (empty)
        {
            
            
            StorageReplicatedMergeTree::Status status;
            getStatus(status,  false);
            if (status.queue.inserts_in_queue)
                empty = false;
        }
        auto backup = restorer.getBackup();
        if (!empty && backup->hasFiles(data_path_in_backup))
            restorer.throwTableIsNotEmpty(getStorageID());
    }
    restorePartsFromBackup(restorer, data_path_in_backup, partitions);
}
void StorageReplicatedMergeTree::attachRestoredParts(MutableDataPartsVector && parts)