Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

{
    renamed = true;
    for (size_t i = 0; i < old_and_new_names.size(); ++i)
    {
        try
        {
            const auto & [old_name, new_name] = old_and_new_names[i];
            if (old_name.empty() || new_name.empty())
                throw DB::Exception("Empty part name. Most likely it's a bug.", ErrorCodes::INCORRECT_FILE_NAME);
            const auto & [path, disk] = old_part_name_to_path_and_disk[old_name];
            const auto full_path = fs::path(path) / source_dir; 
            disk->moveFile(fs::path(full_path) / old_name, fs::path(full_path) / new_name);
        }
        catch (...)
        {
            old_and_new_names.resize(i);
            LOG_WARNING(storage.log, "Cannot rename parts to perform operation on them: {}", getCurrentExceptionMessage(false));
            throw;
        }
    }
}
MergeTreeData::PartsTemporaryRename::~PartsTemporaryRename()