**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)
```
<< parts_to_fetch.size() << " missing parts (with " << parts_to_fetch_blocks << " blocks).";
    /** We can automatically synchronize data,
      *  if the ratio of the total number of errors to the total number of parts (minimum - on the local filesystem or in ZK)
      *  is no more than some threshold (for example 50%).
      *
      * A large ratio of mismatches in the data on the filesystem and the expected data
      *  may indicate a configuration error (the server accidentally connected as a replica not from right shard).
      * In this case, the protection mechanism does not allow the server to start.
      */
    UInt64 total_rows_on_filesystem = 0;
    for (const auto & part : parts)
        total_rows_on_filesystem += part->rows_count;
    const auto storage_settings_ptr = getSettings();
    bool insane = unexpected_parts_rows > total_rows_on_filesystem * storage_settings_ptr->replicated_max_ratio_of_wrong_parts;
    if (insane && !skip_sanity_checks)
    {
        std::stringstream why;
        why << "The local set of parts of table " << getStorageID().getNameForLogs() << " doesn't look like the set of parts "
            << "in ZooKeeper: "
            << formatReadableQuantity(unexpected_parts_rows) << " rows of " << formatReadableQuantity(total_rows_on_filesystem)
            << " total rows in filesystem are suspicious.";
        throw Exception(why.str() + " " + sanity_report.str(), ErrorCodes::TOO_MANY_UNEXPECTED_DATA_PARTS);
    }
    if (unexpected_parts_nonnew_rows > 0)
        LOG_WARNING(log, sanity_report.str());
    
    std::vector<std::future<Coordination::ExistsResponse>> exists_futures;
    exists_futures.reserve(parts_to_fetch.size());
    for (const String & part_name : parts_to_fetch)
    {