**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)
```
"Source replica {} will try to merge this part first", entry.new_part_name,
            LocalDateTime(entry.create_time + storage_settings_ptr->try_fetch_recompressed_part_timeout.totalSeconds()), entry.source_replica);
        return false;
    }
    DataPartsVector parts;
    bool have_all_parts = true;
    for (const String & name : entry.source_parts)
    {
        DataPartPtr part = getActiveContainingPart(name);
        if (!part)
        {
            have_all_parts = false;
            break;
        }
        if (part->name != name)
        {
            LOG_WARNING(log, "Part {} is covered by {} but should be merged into {}. This shouldn't happen often.", name, part->name, entry.new_part_name);
            have_all_parts = false;
            break;
        }
        parts.push_back(part);
    }
    if (!have_all_parts)
    {
        
        LOG_DEBUG(log, "Don't have all parts for merge {}; will try to fetch it instead", entry.new_part_name);
        return false;
    }
    else if (entry.create_time + storage_settings_ptr->prefer_fetch_merged_part_time_threshold.totalSeconds() <= time(nullptr))
    {