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. - 

{
    old_and_new_names.push_back({old_name, new_name});
    for (const auto & [path, disk] : storage.getRelativeDataPathsWithDisks())
    {
        for (auto it = disk->iterateDirectory(fs::path(path) / source_dir); it->isValid(); it->next())
        {
            if (it->name() == old_name)
            {
                old_part_name_to_path_and_disk[old_name] = {path, disk};
                break;
            }
        }
    }
}
void MergeTreeData::PartsTemporaryRename::tryRenameAll()