**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 auto storage_settings_ptr = getSettings();
    return storage_settings_ptr->index_granularity_bytes != 0 &&
        (storage_settings_ptr->enable_mixed_granularity_parts ||
            (!has_non_adaptive_index_granularity_parts && !other_replicas_fixed_granularity));
}
MutationCommands StorageReplicatedMergeTree::getFirtsAlterMutationCommandsForPart(const DataPartPtr & part) const
{
    return queue.getFirstAlterMutationCommandsForPart(part);
}
void StorageReplicatedMergeTree::startBackgroundMovesIfNeeded()
{
    if (areBackgroundMovesNeeded() && !move_parts_task_handle)
    {
        auto & pool = global_context.getBackgroundMovePool();
        move_parts_task_handle = pool.createTask([this] { return movePartsTask(); });
        pool.startTask(move_parts_task_handle);
    }
}
}