**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)
```
int new_metadata_version = metadata_version + 1;
        alter_entry->type = LogEntry::ALTER_METADATA;
        alter_entry->source_replica = replica_name;
        alter_entry->metadata_str = new_metadata_str;
        alter_entry->columns_str = new_columns_str;
        alter_entry->alter_version = new_metadata_version;
        alter_entry->create_time = time(nullptr);
        auto maybe_mutation_commands = commands.getMutationCommands(
            *current_metadata, query_context.getSettingsRef().materialize_ttl_after_modify, query_context);
        alter_entry->have_mutation = !maybe_mutation_commands.empty();
        ops.emplace_back(zkutil::makeCreateRequest(
            zookeeper_path + "/log/log-", alter_entry->toString(), zkutil::CreateMode::PersistentSequential));
        std::optional<EphemeralLocksInAllPartitions> lock_holder;
        
        
        if (alter_entry->have_mutation)
        {
            String mutations_path = zookeeper_path + "/mutations";
            ReplicatedMergeTreeMutationEntry mutation_entry;
            mutation_entry.source_replica = replica_name;
            mutation_entry.commands = maybe_mutation_commands;
            mutation_entry.alter_version = new_metadata_version;
            Coordination::Stat mutations_stat;
            zookeeper->get(mutations_path, &mutations_stat);
            lock_holder.emplace(
                zookeeper_path + "/block_numbers", "block-", zookeeper_path + "/temp", *zookeeper);
            for (const auto & lock : lock_holder->getLocks())
                mutation_entry.block_numbers[lock.partition_id] = lock.number;