**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)
```
catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
            throw;
        }
    });
    
    bool need_sleep = !res && (entry->last_attempt_time - prev_attempt_time < 10);
    
    return need_sleep ? BackgroundProcessingPoolTaskResult::ERROR : BackgroundProcessingPoolTaskResult::SUCCESS;
}
bool StorageReplicatedMergeTree::partIsAssignedToBackgroundOperation(const DataPartPtr & part) const
{
    return queue.isVirtualPart(part);
}
BackgroundProcessingPoolTaskResult StorageReplicatedMergeTree::movePartsTask()
{
    try
    {
        if (!selectPartsAndMove())
            return BackgroundProcessingPoolTaskResult::NOTHING_TO_DO;
        return BackgroundProcessingPoolTaskResult::SUCCESS;
    }
    catch (...)
    {
        tryLogCurrentException(log);
        return BackgroundProcessingPoolTaskResult::ERROR;
    }
}
void StorageReplicatedMergeTree::mergeSelectingTask()