**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)
```
StorageInMemoryMetadata future_metadata = getInMemoryMetadata();
        commands.apply(future_metadata, query_context);
        changeSettings(future_metadata.settings_changes, table_lock_holder);
        DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(query_context, table_id, future_metadata);
        return;
    }
    auto ast_to_str = [](ASTPtr query) -> String
    {
        if (!query)
            return "";
        return queryToString(query);
    };
    auto zookeeper = getZooKeeper();
    std::optional<ReplicatedMergeTreeLogEntryData> alter_entry;
    std::optional<String> mutation_znode;
    while (true)
    {
        
        alter_entry.emplace();
        mutation_znode.reset();
        
        if (is_readonly)
            throw Exception("Can't ALTER readonly table", ErrorCodes::TABLE_IS_READ_ONLY);
        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(*future_metadata.sorting_key.expression_list_ast);