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. - 

candidate.prewhere_info->remove_prewhere_column = prewhere_info->remove_prewhere_column;
            /
            candidate.prewhere_info->row_level_column_name = prewhere_info->row_level_column_name;
            candidate.prewhere_info->need_filter = prewhere_info->need_filter;
            auto prewhere_actions = prewhere_info->prewhere_actions->clone();
            auto prewhere_required_columns = required_columns;
            /
            for (const auto & column : prewhere_actions->getResultColumns())
                required_columns.erase(column.name);
            /
            /
            prewhere_required_columns = prewhere_actions->foldActionsByProjection(
                prewhere_required_columns, projection.sample_block_for_keys, prewhere_info->prewhere_column_name, false);
            /
            /
            if (prewhere_required_columns.empty())
                return false;
            candidate.prewhere_info->prewhere_actions = std::make_shared<ExpressionActions>(prewhere_actions, actions_settings);
            if (prewhere_info->row_level_filter_actions)
            {
                auto row_level_filter_actions = prewhere_info->row_level_filter_actions->clone();
                prewhere_required_columns = row_level_filter_actions->foldActionsByProjection(
                    prewhere_required_columns, projection.sample_block_for_keys, prewhere_info->row_level_column_name, false);
                /
                if (prewhere_required_columns.empty())
                    return false;
                candidate.prewhere_info->row_level_filter
                    = std::make_shared<ExpressionActions>(row_level_filter_actions, actions_settings);
            }
            if (prewhere_info->alias_actions)