**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)
```
PartLog::addNewParts(global_context, res_parts, watch.elapsed());
    }
    catch (...)
    {
        PartLog::addNewParts(global_context, res_parts, watch.elapsed(), ExecutionStatus::fromCurrentException());
        throw;
    }
    tryRemovePartsFromZooKeeperWithRetries(parts_to_remove);
    res_parts.clear();
    parts_to_remove.clear();
    cleanup_thread.wakeup();
    return true;
}
void StorageReplicatedMergeTree::cloneReplica(const String & source_replica, Coordination::Stat source_is_lost_stat, zkutil::ZooKeeperPtr & zookeeper)
{
    String source_path = zookeeper_path + "/replicas/" + source_replica;
    /** TODO: it will be deleted! (It is only to support old version of CH server).
      * In current code, the replica is created in single transaction.
      * If the reference/master replica is not yet fully created, let's wait.
      */
    while (!zookeeper->exists(source_path + "/columns"))
    {
        LOG_INFO(log, "Waiting for replica {} to be fully created", source_path);
        zkutil::EventPtr event = std::make_shared<Poco::Event>();
        if (zookeeper->exists(source_path + "/columns", nullptr, event))
        {
            LOG_WARNING(log, "Oops, a watch has leaked");
            break;
        }
        event->wait();