**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)
```
continue;
        else if (response.error == Coordination::Error::ZNONODE)
        {
            LOG_DEBUG(log, "There is no part {} in ZooKeeper, it was only in filesystem", part_names[i]);
            continue;
        }
        else if (Coordination::isHardwareError(response.error))
        {
            if (parts_should_be_retried)
                parts_should_be_retried->insert(part_names[i]);
            continue;
        }
        else
            LOG_WARNING(log, "Cannot remove part {} from ZooKeeper: {}", part_names[i], Coordination::errorMessage(response.error));
    }
}
void StorageReplicatedMergeTree::clearBlocksInPartition(
    zkutil::ZooKeeper & zookeeper, const String & partition_id, Int64 min_block_num, Int64 max_block_num)
{
    Strings blocks;
    if (Coordination::Error::ZOK != zookeeper.tryGetChildren(zookeeper_path + "/blocks", blocks))
        throw Exception(zookeeper_path + "/blocks doesn't exist", ErrorCodes::NOT_FOUND_NODE);
    String partition_prefix = partition_id + "_";
    zkutil::AsyncResponses<Coordination::GetResponse> get_futures;
    for (const String & block_id : blocks)
    {
        if (startsWith(block_id, partition_prefix))
        {
            String path = zookeeper_path + "/blocks/" + block_id;
            get_futures.emplace_back(path, zookeeper.asyncTryGet(path));