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. - 

commands.apply(new_metadata, getContext());
    
    NameSet columns_alter_type_forbidden;
    
    
    NameSet columns_alter_type_metadata_only;
    
    NameSet columns_alter_type_check_safe_for_partition;
    if (old_metadata.hasPartitionKey())
    {
        
        auto partition_key_expr = old_metadata.getPartitionKey().expression;
        for (const auto & action : partition_key_expr->getActions())
        {
            for (const auto * child : action.node->children)
                columns_alter_type_forbidden.insert(child->result_name);
        }
        
        for (const String & col : partition_key_expr->getRequiredColumns())
            columns_alter_type_check_safe_for_partition.insert(col);
    }
    for (const auto & index : old_metadata.getSecondaryIndices())
    {
        for (const String & col : index.expression->getRequiredColumns())
            columns_alter_type_forbidden.insert(col);
    }
    if (old_metadata.hasSortingKey())
    {
        auto sorting_key_expr = old_metadata.getSortingKey().expression;
        for (const auto & action : sorting_key_expr->getActions())