**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.
alter_entry.emplace();
        mutation_znode.reset();
        auto current_metadata = getInMemoryMetadataPtr();
        StorageInMemoryMetadata future_metadata = *current_metadata;
        commands.apply(future_metadata, query_context);
        ReplicatedMergeTreeTableMetadata future_metadata_in_zk(*this, current_metadata);
        if (ast_to_str(future_metadata.sorting_key.definition_ast) != ast_to_str(current_metadata->sorting_key.definition_ast))
        {
            
            
            
            
            future_metadata_in_zk.sorting_key = serializeAST(*extractKeyExpressionList(future_metadata.sorting_key.definition_ast));
        }
        if (ast_to_str(future_metadata.sampling_key.definition_ast) != ast_to_str(current_metadata->sampling_key.definition_ast))
            future_metadata_in_zk.sampling_expression = serializeAST(*extractKeyExpressionList(future_metadata.sampling_key.definition_ast));
        if (ast_to_str(future_metadata.partition_key.definition_ast) != ast_to_str(current_metadata->partition_key.definition_ast))
            future_metadata_in_zk.partition_key = serializeAST(*extractKeyExpressionList(future_metadata.partition_key.definition_ast));
        if (ast_to_str(future_metadata.table_ttl.definition_ast) != ast_to_str(current_metadata->table_ttl.definition_ast))
        {
            if (future_metadata.table_ttl.definition_ast)
                future_metadata_in_zk.ttl_table = serializeAST(*future_metadata.table_ttl.definition_ast);
            else 
                future_metadata_in_zk.ttl_table = "";
        }
        String new_indices_str = future_metadata.secondary_indices.toString();
        if (new_indices_str != current_metadata->secondary_indices.toString())
            future_metadata_in_zk.skip_indices = new_indices_str;
        String new_projections_str = future_metadata.projections.toString();