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 (!part->is_duplicate)
            parts_to_delete_completely.emplace_back(part);
        else
            parts_to_delete_only_from_filesystem.emplace_back(part);
    }
    parts.clear();
    
    if (!parts_to_delete_only_from_filesystem.empty())
    {
        removePartsFromFilesystem(parts_to_delete_only_from_filesystem);
        removePartsFinally(parts_to_delete_only_from_filesystem);
        LOG_DEBUG(log, "Removed {} old duplicate parts", parts_to_delete_only_from_filesystem.size());
    }
    
    NameSet part_names_to_retry_deletion;
    try
    {
        Strings part_names_to_delete_completely;
        for (const auto & part : parts_to_delete_completely)
            part_names_to_delete_completely.emplace_back(part->name);
        LOG_DEBUG(log, "Removing {} old parts from ZooKeeper", parts_to_delete_completely.size());
        removePartsFromZooKeeper(zookeeper, part_names_to_delete_completely, &part_names_to_retry_deletion);
    }
    catch (...)
    {
        LOG_ERROR(log, "There is a problem with deleting parts from ZooKeeper: {}", getCurrentExceptionMessage(true));
    }
    
    auto num_reliably_deleted_parts = parts_to_delete_completely.size() - part_names_to_retry_deletion.size();
    LOG_DEBUG(log, "Removed {} old parts from ZooKeeper. Removing them from filesystem.", num_reliably_deleted_parts);