Hello,

so I'm working to integrate CatBoost model with our ClickHouse cluster. It appears that, whenever we create a new XML model file in the configured path, the **first** call to the new model never returns (this is true both of the HTTP interface as well as via clickhouse-client).

Independently from whether the locked statement gets cancelled, subsequent calls succeed immediately.

```
16e8bdb5bbb9 :) select version();

SELECT version()

┌─version()─┐
│ 20.6.3.28 │
└───────────┘

1 rows in set. Elapsed: 0.006 sec.
```

Is this known behavior? If it helps, my models look like this:

```
<models>
    <model>
        <type>catboost</type>
        <name>ctr_predictor</name>
        <path>/catboost/my_predictor</path>
        <lifetime>0</lifetime>
    </model>
</models>
```
{
        setThreadName("ExterLdrReload");
        std::unique_lock lock{mutex};
        auto pred = [this] { return !enabled; };
        while (!event.wait_for(lock, std::chrono::seconds(check_period_sec), pred))
        {
            lock.unlock();
            loading_dispatcher.setConfiguration(config_files_reader.read());
            loading_dispatcher.reloadOutdated();
            lock.lock();
        }
    }
    LoadablesConfigReader & config_files_reader;
    LoadingDispatcher & loading_dispatcher;
    mutable std::mutex mutex;
    bool enabled = false;
    ThreadFromGlobalPool thread;
    std::condition_variable event;
};
ExternalLoader::ExternalLoader(const String & type_name_, Poco::Logger * log_)