**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)
```
{
        
        
        String replica = findReplicaHavingPart(entry.new_part_name, true);    
        if (!replica.empty())
        {
            LOG_DEBUG(log, "Prefer to fetch {} from replica {}", entry.new_part_name, replica);
            return false;
        }
    }
    MergeTreePartInfo new_part_info = MergeTreePartInfo::fromPartName(
        entry.new_part_name, format_version);
    MutationCommands commands = queue.getMutationCommands(source_part, new_part_info.mutation);
    
    
    ReservationPtr reserved_space = reserveSpace(estimated_space_for_result, source_part->volume);
    auto table_lock = lockForShare(
            RWLockImpl::NO_QUERY, storage_settings_ptr->lock_acquire_timeout_for_background_operations);
    StorageMetadataPtr metadata_snapshot = getInMemoryMetadataPtr();
    MutableDataPartPtr new_part;
    Transaction transaction(*this);
    FutureMergedMutatedPart future_mutated_part;
    future_mutated_part.parts.push_back(source_part);
    future_mutated_part.part_info = new_part_info;
    future_mutated_part.name = entry.new_part_name;
    future_mutated_part.updatePath(*this, reserved_space);
    future_mutated_part.type = source_part->getType();
    auto table_id = getStorageID();
    MergeList::EntryPtr merge_entry = global_context.getMergeList().insert(
        table_id.database_name, table_id.table_name, future_mutated_part);