**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.
const MutationCommands & commands, ContextPtr query_context, const zkutil::ZooKeeperPtr & zookeeper) const
{
    const std::set<String> mutation_affected_partition_ids = getPartitionIdsAffectedByCommands(commands, query_context);
    if (mutation_affected_partition_ids.size() == 1)
    {
        const auto & affected_partition_id = *mutation_affected_partition_ids.cbegin();
        auto block_number_holder = allocateBlockNumber(affected_partition_id, zookeeper);
        if (!block_number_holder.has_value())
            return {};
        auto block_number = block_number_holder->getNumber();  
        return {{{affected_partition_id, block_number}}, std::move(block_number_holder)};
    }
    else
    {
        
        EphemeralLocksInAllPartitions lock_holder(
            fs::path(zookeeper_path) / "block_numbers", "block-", fs::path(zookeeper_path) / "temp", *zookeeper);
        PartitionBlockNumbersHolder::BlockNumbersType block_numbers;
        for (const auto & lock : lock_holder.getLocks())
        {
            if (mutation_affected_partition_ids.empty() || mutation_affected_partition_ids.contains(lock.partition_id))
                block_numbers[lock.partition_id] = lock.number;
        }
        return {std::move(block_numbers), std::move(lock_holder)};
    }
}
void StorageReplicatedMergeTree::alter(