**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 mutation_pointer = zookeeper_path + "/replicas/" + replica + "/mutation_pointer";
            std::string mutation_pointer_value;
            Coordination::Stat get_stat;
            
            if (!zookeeper->tryGet(mutation_pointer, mutation_pointer_value, &get_stat, wait_event))
            {
                LOG_WARNING(log, "Replica {} was removed", replica);
                break;
            }
            else if (mutation_pointer_value >= mutation_id) 
                break;                                      
            
            if (wait_event->tryWait(1000))
                break;
            auto mutation_status = queue.getIncompleteMutationsStatus(mutation_id);
            if (!mutation_status || !mutation_status->latest_fail_reason.empty())
                break;
        }
        
        
        if (!getZooKeeper()->exists(zookeeper_path + "/mutations/" + mutation_id))
        {
            throw Exception(ErrorCodes::UNFINISHED, "Mutation {} was killed, manually removed or table was dropped", mutation_id);
        }
        
        std::set<String> mutation_ids;
        mutation_ids.insert(mutation_id);
        auto mutation_status = queue.getIncompleteMutationsStatus(mutation_id, &mutation_ids);
        checkMutationStatus(mutation_status, mutation_ids);