I see this in logs on server shutdown:

```
2022.01.30 02:51:22.856463 [ 755777 ] {} <Error> void DB::MergeTreeBackgroundExecutor<DB::MergeMutateRuntimeQueue>::routine(DB::TaskRuntimeDataPtr) [Queue = DB::MergeMutateRuntimeQueue]: Code: 236. DB::Exception: Cancelled merging parts.
```

but it's not an error.

Log level should be changed and it should not show stack trace.
{
public:
    TaskRuntimeDataPtr pop()
    {
        auto result = std::move(queue.front());
        queue.pop_front();
        return result;
    }
    void push(TaskRuntimeDataPtr item) { queue.push_back(std::move(item));}
    void remove(StorageID id)
    {
        auto it = std::remove_if(queue.begin(), queue.end(),
            [&] (auto item) -> bool { return item->task->getStorageID() == id; });
        queue.erase(it, queue.end());
    }
    void setCapacity(size_t count) { queue.set_capacity(count); }
    bool empty() { return queue.empty(); }
private:
    boost::circular_buffer<TaskRuntimeDataPtr> queue{0};
};
class MergeMutateRuntimeQueue