**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)
```
UInt64 log_index = parse<UInt64>(entry.znode_name.substr(entry.znode_name.size() - 10));
        log_node_name = entry.znode_name;
        LOG_DEBUG(log, "Waiting for {} to pull {} to queue", replica, log_node_name);
        
        while (!stop_waiting())
        {
            zkutil::EventPtr event = std::make_shared<Poco::Event>();
            String log_pointer = getZooKeeper()->get(zookeeper_path + "/replicas/" + replica + "/log_pointer", nullptr, event);
            if (!log_pointer.empty() && parse<UInt64>(log_pointer) > log_index)
                break;
            if (wait_for_non_active)
                event->wait();
            else
                event->tryWait(event_wait_timeout_ms);
        }
    }
    else if (startsWith(entry.znode_name, "queue-"))
    {
        /** In this case, the number of `log` node is unknown. You need look through everything from `log_pointer` to the end,
          *  looking for a node with the same content. And if we do not find it - then the replica has already taken this entry in its queue.
          */
        String log_pointer = getZooKeeper()->get(zookeeper_path + "/replicas/" + replica + "/log_pointer");
        Strings log_entries = getZooKeeper()->getChildren(zookeeper_path + "/log");
        UInt64 log_index = 0;
        bool found = false;
        for (const String & log_entry_name : log_entries)
        {
            log_index = parse<UInt64>(log_entry_name.substr(log_entry_name.size() - 10));
            if (!log_pointer.empty() && log_index < parse<UInt64>(log_pointer))
                continue;