**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.
ReplicatedMergeTreeMutationEntry mutation_entry;
    mutation_entry.source_replica = replica_name;
    mutation_entry.commands = commands;
    const String mutations_path = fs::path(zookeeper_path) / "mutations";
    const auto zookeeper = getZooKeeper();
    
    
    
    while (true)
    {
        Coordination::Stat mutations_stat;
        zookeeper->get(mutations_path, &mutations_stat);
        PartitionBlockNumbersHolder partition_block_numbers_holder =
                allocateBlockNumbersInAffectedPartitions(mutation_entry.commands, query_context, zookeeper);
        mutation_entry.block_numbers = partition_block_numbers_holder.getBlockNumbers();
        mutation_entry.create_time = time(nullptr);
        
        
        
        Coordination::Requests requests;
        requests.emplace_back(zkutil::makeSetRequest(mutations_path, String(), mutations_stat.version));