**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 String & path_created =
                dynamic_cast<const Coordination::CreateResponse *>(responses[1].get())->path_created;
            entry.znode_name = path_created.substr(path_created.find_last_of('/') + 1);
            LOG_TRACE(log, "Created mutation with ID {}", entry.znode_name);
            break;
        }
        else if (rc == Coordination::Error::ZBADVERSION)
        {
            LOG_TRACE(log, "Version conflict when trying to create a mutation node, retrying...");
            continue;
        }
        else
            throw Coordination::Exception("Unable to create a mutation znode", rc);
    }
    waitMutation(entry.znode_name, query_context.getSettingsRef().mutations_sync);
}
void StorageReplicatedMergeTree::waitMutation(const String & znode_name, size_t mutations_sync) const
{
    if (!mutations_sync)
        return;
    
    auto zookeeper = getZooKeeper();
    Strings replicas;
    if (mutations_sync == 2) 
        replicas = zookeeper->getChildren(zookeeper_path + "/replicas");
    else if (mutations_sync == 1) 
        replicas.push_back(replica_name);
    waitMutationToFinishOnReplicas(replicas, znode_name);
}
std::vector<MergeTreeMutationStatus> StorageReplicatedMergeTree::getMutationsStatus() const