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 (candidate.desc->type == ProjectionDescription::Type::Normal)
                {
                    selectBestProjection(
                        reader,
                        metadata_snapshot,
                        query_info,
                        analysis_result.required_columns,
                        candidate,
                        query_context,
                        max_added_blocks.get(),
                        settings,
                        parts,
                        selected_candidate,
                        min_sum_marks);
                }
            }
        }
        if (!selected_candidate)
            return false;
        if (selected_candidate->desc->type == ProjectionDescription::Type::Aggregate)
        {
            selected_candidate->aggregation_keys = select.getQueryAnalyzer()->aggregationKeys();
            selected_candidate->aggregate_descriptions = select.getQueryAnalyzer()->aggregates();
        }
        query_info.projection = std::move(*selected_candidate);
        return true;
    }
    return false;
}
QueryProcessingStage::Enum MergeTreeData::getQueryProcessingStage(