**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)
```
if (part->getBytesOnDisk() > max_source_part_size_for_mutation)
                        continue;
                    std::optional<std::pair<Int64, int>> desired_mutation_version = merge_pred.getDesiredMutationVersion(part);
                    if (!desired_mutation_version)
                        continue;
                    create_result = createLogEntryToMutatePart(*part,
                        desired_mutation_version->first, desired_mutation_version->second, merge_pred.getVersion());
                    if (create_result == CreateMergeEntryResult::Ok)
                        break;
                }
            }
        }
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
    }
    if (!is_leader)
        return;
    if (create_result != CreateMergeEntryResult::Ok
        && create_result != CreateMergeEntryResult::LogUpdated)
    {
        merge_selecting_task->scheduleAfter(MERGE_SELECTING_SLEEP_MS);
    }
    else
    {
        merge_selecting_task->schedule();
    }
}
void StorageReplicatedMergeTree::mutationsFinalizingTask()