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>
```
{
        if constexpr (std::is_same_v<ReturnType, ExternalLoader::LoadResult>)
        {
            ExternalLoader::LoadResult res;
            res.name = name;
            return res;
        }
        else
        {
            static_assert(std::is_same_v<ReturnType, ExternalLoader::LoadablePtr>);
            return nullptr;
        }
    }
    
    
    struct LoadingGuardForAsyncLoad
    {
        std::unique_lock<std::mutex> lock;
        LoadingGuardForAsyncLoad(bool async, std::mutex & mutex)
        {
            if (async)
                lock = std::unique_lock(mutex);
        }
    };
}
/** Reads configurations from configuration repository and parses it.
  */
class ExternalLoader::LoadablesConfigReader : private boost::noncopyable