**Describe the bug**
The ClickHouse client freezes when executing `ALTER TABLE` with `ADD COLUMN` and `MODIFY ORDER` on a table built using Replicated Versioned Collapsing Merge Tree engine.

**How to reproduce**
* ClickHouse client version 20.10.1.1.
Connecting to localhost:9000 as user default.
Connected to ClickHouse server version 20.10.1 revision 54440.

* `CREATE TABLE` statements
```
SHOW CREATE TABLE table0

┌─statement──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.table0
(
    `d` Date,
    `a` String,
    `b` UInt8,
    `x` String,
    `y` Int8,
    `version` UInt64,
    `sign` Int8 DEFAULT 1
)
ENGINE = ReplicatedVersionedCollapsingMergeTree('/clickhouse/tables/{shard}/table0', '{replica}', sign, version)
PARTITION BY y
ORDER BY d
SETTINGS index_granularity = 8192 │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

1 rows in set. Elapsed: 0.004 sec.
```

* Queries to run that lead to unexpected result
```
ALTER TABLE table0 ADD COLUMN order UInt32, MODIFY ORDER BY (d, order)
```
ColumnsDescription new_columns, const ReplicatedMergeTreeTableMetadata::Diff & metadata_diff)
{
    StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
    StorageInMemoryMetadata old_metadata = getInMemoryMetadata();
    if (new_columns != new_metadata.columns)
    {
        new_metadata.columns = new_columns;
        new_metadata.column_ttls_by_name.clear();
        for (const auto & [name, ast] : new_metadata.columns.getColumnTTLs())
        {
            auto new_ttl_entry = TTLDescription::getTTLFromAST(ast, new_metadata.columns, global_context, new_metadata.primary_key);
            new_metadata.column_ttls_by_name[name] = new_ttl_entry;
        }
        
        if (new_metadata.partition_key.definition_ast != nullptr)
            new_metadata.partition_key.recalculateWithNewColumns(new_metadata.columns, global_context);
    }
    if (!metadata_diff.empty())
    {
        auto parse_key_expr = [](const String & key_expr)
        {
            ParserNotEmptyExpressionList parser(false);
            auto new_sorting_key_expr_list = parseQuery(parser, key_expr, 0, DBMS_DEFAULT_MAX_PARSER_DEPTH);
            ASTPtr order_by_ast;
            if (new_sorting_key_expr_list->children.size() == 1)
                order_by_ast = new_sorting_key_expr_list->children[0];