**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 executeMetadataAlter(entry);
    }
    else
    {
        throw Exception("Unexpected log entry type: " + toString(static_cast<int>(entry.type)), ErrorCodes::LOGICAL_ERROR);
    }
    if (do_fetch)
        return executeFetch(entry);
    return true;
}
bool StorageReplicatedMergeTree::tryExecuteMerge(const LogEntry & entry)
{
    
    {
        std::stringstream source_parts_msg;
        for (auto i : ext::range(0, entry.source_parts.size()))
            source_parts_msg << (i != 0 ? ", " : "") << entry.source_parts[i];
        LOG_TRACE(log, "Executing log entry to merge parts {} to {}", source_parts_msg.str(), entry.new_part_name);
    }
    const auto storage_settings_ptr = getSettings();
    if (storage_settings_ptr->always_fetch_merged_part)
    {
        LOG_INFO(log, "Will fetch part {} because setting 'always_fetch_merged_part' is true", entry.new_part_name);
        return false;
    }
    if (entry.merge_type == MergeType::TTL_RECOMPRESS &&
        (time(nullptr) - entry.create_time) <= storage_settings_ptr->try_fetch_recompressed_part_timeout.totalSeconds() &&
        entry.source_replica != replica_name)
    {
        LOG_INFO(log, "Will try to fetch part {} until '{}' because this part assigned to recompression merge. "