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. - 

LOG_WARNING(log, "currently_submerging_big_parts contains duplicates. JBOD might lose balance");
                    else
                        currently_submerging_big_parts.insert(part);
                }
                /
                tagger_ptr->emplace(*this, part_name, std::move(covered_parts), log);
            }
        }
    }
    catch (...)
    {
        LOG_DEBUG(log, "JBOD balancer encounters an error. Fallback to random disk selection");
        tryLogCurrentException(log);
    }
    return reserved_space;
}
CurrentlySubmergingEmergingTagger::~CurrentlySubmergingEmergingTagger()
{
    std::lock_guard lock(storage.currently_submerging_emerging_mutex);
    for (const auto & part : submerging_parts)
    {
        if (!storage.currently_submerging_big_parts.count(part))
            LOG_WARNING(log, "currently_submerging_big_parts doesn't contain part {} to erase. This is a bug", part->name);
        else
            storage.currently_submerging_big_parts.erase(part);
    }
    storage.currently_emerging_big_parts.erase(emerging_part_name);
}
}