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. - 

{
            for (const auto * child : action.node->children)
                columns_alter_type_forbidden.insert(child->result_name);
        }
        for (const String & col : sorting_key_expr->getRequiredColumns())
            columns_alter_type_metadata_only.insert(col);
        
        
    }
    if (!merging_params.sign_column.empty())
        columns_alter_type_forbidden.insert(merging_params.sign_column);
    
    NameSet columns_in_keys;
    columns_in_keys.insert(columns_alter_type_forbidden.begin(), columns_alter_type_forbidden.end());
    columns_in_keys.insert(columns_alter_type_metadata_only.begin(), columns_alter_type_metadata_only.end());
    columns_in_keys.insert(columns_alter_type_check_safe_for_partition.begin(), columns_alter_type_check_safe_for_partition.end());
    NameSet dropped_columns;
    std::map<String, const IDataType *> old_types;
    for (const auto & column : old_metadata.getColumns().getAllPhysical())
        old_types.emplace(column.name, column.type.get());
    NamesAndTypesList columns_to_check_conversion;
    auto name_deps = getDependentViewsByColumn(local_context);
    for (const AlterCommand & command : commands)
    {
        
        if (command.partition)
        {
            getPartitionIDFromQuery(command.partition, getContext());
        }
        if (command.column_name == merging_params.version_column)