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. - 

for (const auto & part : covered_parts)
                submerging_big_parts_from_partition.insert(part->name);
            for (const auto & part : getDataPartsStateRange(MergeTreeData::DataPartState::Committed))
            {
                if (part->isStoredOnDisk() && part->getBytesOnDisk() >= min_bytes_to_rebalance_partition_over_jbod
                    && part_info.partition_id == part->info.partition_id)
                {
                    auto name = part->volume->getDisk()->getName();
                    auto it = disk_occupation.find(name);
                    if (it != disk_occupation.end())
                    {
                        if (submerging_big_parts_from_partition.find(part->name) == submerging_big_parts_from_partition.end())
                        {
                            it->second += part->getBytesOnDisk();
                            disk_parts_for_logging[name].push_back(formatReadableSizeWithBinarySuffix(part->getBytesOnDisk()));
                            committed_big_parts_from_partition.insert(part->name);
                        }
                        else
                        {
                            disk_parts_for_logging[name].push_back(formatReadableSizeWithBinarySuffix(part->getBytesOnDisk()) + " (submerging)");
                        }
                    }
                    else
                    {
                        /
                    }
                }
            }
        }
        for (const auto & [name, emerging_part] : currently_emerging_big_parts)