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. - 

: current_time - settings->temporary_directories_lifetime.totalSeconds();
    
    for (const auto & [path, disk] : getRelativeDataPathsWithDisks())
    {
        for (auto it = disk->iterateDirectory(path); it->isValid(); it->next())
        {
            if (startsWith(it->name(), "tmp_"))
            {
                try
                {
                    if (disk->isDirectory(it->path()) && isOldPartDirectory(disk, it->path(), deadline))
                    {
                        LOG_WARNING(log, "Removing temporary directory {}", fullPath(disk, it->path()));
                        disk->removeRecursive(it->path());
                    }
                }
                catch (const fs::filesystem_error & e)
                {
                    if (e.code() == std::errc::no_such_file_or_directory)
                    {
                        
                    }
                    else
                        throw;
                }
            }
        }
    }
}
MergeTreeData::DataPartsVector MergeTreeData::grabOldParts(bool force)