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. - 

{
                auto alias_actions = prewhere_info->alias_actions->clone();
                /
                prewhere_required_columns
                    = alias_actions->foldActionsByProjection(prewhere_required_columns, projection.sample_block_for_keys, {}, false);
                /
                /
                if (prewhere_required_columns.empty())
                    return false;
                candidate.prewhere_info->alias_actions = std::make_shared<ExpressionActions>(alias_actions, actions_settings);
            }
            required_columns.insert(prewhere_required_columns.begin(), prewhere_required_columns.end());
        }
        bool match = true;
        for (const auto & column : required_columns)
        {
            /
            if (!source_block.has(column))
            {
                match = false;
                break;
            }
        }
        return match;
    };
    for (const auto & projection : metadata_snapshot->projections)
    {
        ProjectionCandidate candidate{};
        candidate.desc = &projection;
        if (projection.type == ProjectionDescription::Type::Aggregate && analysis_result.need_aggregate && can_use_aggregate_projection)