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 & [p_name, projection_part] : part->getProjectionParts())
                        res.push_back(projection_part);
                }
            }
            else
            {
                std::swap(buf, res);
                res.clear();
                std::merge(range.begin(), range.end(), buf.begin(), buf.end(), std::back_inserter(res), LessDataPart()); /
            }
        }
        if (out_states != nullptr)
        {
            out_states->resize(res.size());
            if (require_projection_parts)
            {
                for (size_t i = 0; i < res.size(); ++i)
                    (*out_states)[i] = res[i]->getParentPart()->getState();
            }
            else
            {
                for (size_t i = 0; i < res.size(); ++i)
                    (*out_states)[i] = res[i]->getState();
            }
        }
    }
    return res;
}
MergeTreeData::DataPartsVector