**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)
```
MergeTreeDataFormatVersion format_version)
            : index(index_),
            src_part_name(src_part_name_), src_part_info(MergeTreePartInfo::fromPartName(src_part_name_, format_version)),
            new_part_name(new_part_name_), new_part_info(MergeTreePartInfo::fromPartName(new_part_name_, format_version)),
            checksum_hex(checksum_hex_) {}
        size_t index; 
        String src_part_name;
        MergeTreePartInfo src_part_info;
        String new_part_name;
        MergeTreePartInfo new_part_info;
        String checksum_hex;
        
        MutableDataPartPtr res_part;
        
        MergeTreePartInfo found_new_part_info;
        String found_new_part_name;
        
        DataPartPtr src_table_part;
        
        String replica;
    };
    using PartDescriptionPtr = std::shared_ptr<PartDescription>;
    using PartDescriptions = std::vector<PartDescriptionPtr>;
    PartDescriptions all_parts;
    PartDescriptions parts_to_add;
    DataPartsVector parts_to_remove;