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 (command.type == AlterCommand::MODIFY_TTL && !is_custom_partitioned)
        {
            throw Exception(
                "ALTER MODIFY TTL is not supported for default-partitioned tables created with the old syntax",
                ErrorCodes::BAD_ARGUMENTS);
        }
        if (command.type == AlterCommand::MODIFY_SAMPLE_BY)
        {
            if (!is_custom_partitioned)
                throw Exception(
                    "ALTER MODIFY SAMPLE BY is not supported for default-partitioned tables created with the old syntax",
                    ErrorCodes::BAD_ARGUMENTS);
            checkSampleExpression(new_metadata, getSettings()->compatibility_allow_sampling_expression_not_in_primary_key);
        }
        if (command.type == AlterCommand::ADD_INDEX && !is_custom_partitioned)
        {
            throw Exception(
                "ALTER ADD INDEX is not supported for tables with the old syntax",
                ErrorCodes::BAD_ARGUMENTS);
        }
        if (command.type == AlterCommand::ADD_PROJECTION && !is_custom_partitioned)
        {
            throw Exception(
                "ALTER ADD PROJECTION is not supported for tables with the old syntax",
                ErrorCodes::BAD_ARGUMENTS);
        }
        if (command.type == AlterCommand::RENAME_COLUMN)
        {
            if (columns_in_keys.count(command.column_name))
            {