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_type->canBeUsedAsVersion())
        throw Exception("Cannot alter version column " + backQuoteIfNeed(column_name) +
            " to type " + new_type->getName() +
            " because version column must be of an integer type or of type Date or DateTime"
            , ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
    auto which_new_type = WhichDataType(new_type);
    auto which_old_type = WhichDataType(old_type);
    
    if ((which_old_type.isInt() && !which_new_type.isInt())
        || (which_old_type.isUInt() && !which_new_type.isUInt())
        || (which_old_type.isDate() && !which_new_type.isDate())
        || (which_old_type.isDateTime() && !which_new_type.isDateTime())
        || (which_old_type.isFloat() && !which_new_type.isFloat()))