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 (columns_alter_type_forbidden.count(command.column_name))
                throw Exception("ALTER of key column " + backQuoteIfNeed(command.column_name) + " is forbidden",
                    ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
            if (command.type == AlterCommand::MODIFY_COLUMN)
            {
                if (columns_alter_type_check_safe_for_partition.count(command.column_name))
                {
                    auto it = old_types.find(command.column_name);
                    assert(it != old_types.end());
                    if (!isSafeForKeyConversion(it->second, command.data_type.get()))
                        throw Exception("ALTER of partition key column " + backQuoteIfNeed(command.column_name) + " from type "
                                + it->second->getName() + " to type " + command.data_type->getName()
                                + " is not safe because it can change the representation of partition key",
                            ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
                }
                if (columns_alter_type_metadata_only.count(command.column_name))
                {
                    auto it = old_types.find(command.column_name);
                    assert(it != old_types.end());
                    if (!isSafeForKeyConversion(it->second, command.data_type.get()))
                        throw Exception("ALTER of key column " + backQuoteIfNeed(command.column_name) + " from type "
                                    + it->second->getName() + " to type " + command.data_type->getName()
                                    + " is not safe because it can change the representation of primary key",
                            ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
                }
                if (old_metadata.getColumns().has(command.column_name))
                {
                    columns_to_check_conversion.push_back(
                        new_metadata.getColumns().getPhysical(command.column_name));
                }