**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)
```
{
    const String & source_part_name = entry.source_parts.at(0);
    const auto storage_settings_ptr = getSettings();
    LOG_TRACE(log, "Executing log entry to mutate part {} to {}", source_part_name, entry.new_part_name);
    DataPartPtr source_part = getActiveContainingPart(source_part_name);
    if (!source_part)
    {
        LOG_DEBUG(log, "Source part {} for {} is not ready; will try to fetch it instead", source_part_name, entry.new_part_name);
        return false;
    }
    if (source_part->name != source_part_name)
    {
        throw Exception("Part " + source_part_name + " is covered by " + source_part->name
            + " but should be mutated to " + entry.new_part_name + ". This is a bug.",
            ErrorCodes::LOGICAL_ERROR);
    }
    
    size_t estimated_space_for_result = MergeTreeDataMergerMutator::estimateNeededDiskSpace({source_part});
    if (entry.create_time + storage_settings_ptr->prefer_fetch_merged_part_time_threshold.totalSeconds() <= time(nullptr)
        && estimated_space_for_result >= storage_settings_ptr->prefer_fetch_merged_part_size_threshold)