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 (miss_column)
            throw Exception("Version column " + version_column + " does not exist in table declaration.", ErrorCodes::NO_SUCH_COLUMN_IN_TABLE);
    };
    if (mode == MergingParams::Collapsing)
        check_sign_column(false, "CollapsingMergeTree");
    if (mode == MergingParams::Summing)
    {
        
        for (const auto & column_to_sum : columns_to_sum)
        {
            auto check_column_to_sum_exists = [& column_to_sum](const NameAndTypePair & name_and_type)
            {
                return column_to_sum == Nested::extractTableName(name_and_type.name);
            };
            if (columns.end() == std::find_if(columns.begin(), columns.end(), check_column_to_sum_exists))
                throw Exception(
                        "Column " + column_to_sum + " listed in columns to sum does not exist in table declaration.", ErrorCodes::NO_SUCH_COLUMN_IN_TABLE);
        }
        
        if (metadata.isPartitionKeyDefined())
        {
            auto partition_key_columns = metadata.getPartitionKey().column_names;
            Names names_intersection;
            std::set_intersection(columns_to_sum.begin(), columns_to_sum.end(),
                                  partition_key_columns.begin(), partition_key_columns.end(),
                                  std::back_inserter(names_intersection));
            if (!names_intersection.empty())
                throw Exception("Columns: " + boost::algorithm::join(names_intersection, ", ") +
                " listed both in columns to sum and in partition key. That is not allowed.", ErrorCodes::BAD_ARGUMENTS);