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 (new_partition_key.expression_list_ast->children.empty())
        return;
    checkKeyExpression(*new_partition_key.expression, new_partition_key.sample_block, "Partition", allow_nullable_key);
    
    DataTypes minmax_idx_columns_types = getMinMaxColumnsTypes(new_partition_key);
    
    
    bool has_date_column = false;
    bool has_datetime_column = false;
    for (size_t i = 0; i < minmax_idx_columns_types.size(); ++i)
    {
        if (isDate(minmax_idx_columns_types[i]))
        {
            if (!has_date_column)
            {
                minmax_idx_date_column_pos = i;
                has_date_column = true;
            }
            else
            {
                
                minmax_idx_date_column_pos = -1;
            }
        }
    }
    if (!has_date_column)
    {
        for (size_t i = 0; i < minmax_idx_columns_types.size(); ++i)