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. - 

{
    
    if (!renamed)
        return;
    for (const auto & [old_name, new_name] : old_and_new_names)
    {
        if (old_name.empty())
            continue;
        try
        {
            const auto & [path, disk] = old_part_name_to_path_and_disk[old_name];
            const String full_path = fs::path(path) / source_dir; 
            disk->moveFile(fs::path(full_path) / new_name, fs::path(full_path) / old_name);
        }
        catch (...)
        {
            tryLogCurrentException(__PRETTY_FUNCTION__);
        }
    }
}
MergeTreeData::DataPartsVector MergeTreeData::getActivePartsToReplace(