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. - 

error_parts += (*it)->getNameWithState() + " ";
        };
        auto it_middle = data_parts_by_info.lower_bound(part->info);
        
        if (it_middle != data_parts_by_info.begin())
        {
            auto it = std::prev(it_middle);
            if (part->contains(**it) && is_appropriate_state((*it)->getState()))
            {
                
                if ((*it)->info.min_block != part->info.min_block)
                    update_error(it);
                if ((*it)->getState() != DataPartState::Committed)
                {
                    addPartContributionToColumnSizes(*it);
                    addPartContributionToDataVolume(*it);
                    modifyPartState(it, DataPartState::Committed); 
                }
                pos = (*it)->info.max_block + 1;
                restored.push_back((*it)->name);
            }
            else
                update_error(it);
        }
        else
            error = true;
        
        for (auto it = it_middle; it != data_parts_by_info.end() && part->contains(**it); ++it)
        {
            if ((*it)->info.min_block < pos)