**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)
```
{
    return queue.getMutationsStatus();
}
CancellationCode StorageReplicatedMergeTree::killMutation(const String & mutation_id)
{
    assertNotReadonly();
    zkutil::ZooKeeperPtr zookeeper = getZooKeeper();
    LOG_TRACE(log, "Killing mutation {}", mutation_id);
    auto mutation_entry = queue.removeMutation(zookeeper, mutation_id);
    if (!mutation_entry)
        return CancellationCode::NotFound;
    
    
    
    for (const auto & pair : mutation_entry->block_numbers)
    {
        const String & partition_id = pair.first;
        Int64 block_number = pair.second;
        global_context.getMergeList().cancelPartMutations(partition_id, block_number);
    }
    return CancellationCode::CancelSent;
}
void StorageReplicatedMergeTree::clearOldPartsAndRemoveFromZK()