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 (from->getName() == to->getName())
        return true;
    
    
    
    if (const auto * from_enum8 = typeid_cast<const DataTypeEnum8 *>(from))
    {
        if (const auto * to_enum8 = typeid_cast<const DataTypeEnum8 *>(to))
            return to_enum8->contains(*from_enum8);
        if (typeid_cast<const DataTypeInt8 *>(to))
            return true;    
        return false;
    }
    if (const auto * from_enum16 = typeid_cast<const DataTypeEnum16 *>(from))
    {
        if (const auto * to_enum16 = typeid_cast<const DataTypeEnum16 *>(to))
            return to_enum16->contains(*from_enum16);
        if (typeid_cast<const DataTypeInt16 *>(to))
            return true;    
        return false;
    }
    if (const auto * from_lc = typeid_cast<const DataTypeLowCardinality *>(from))
        return from_lc->getDictionaryType()->equals(*to);
    if (const auto * to_lc = typeid_cast<const DataTypeLowCardinality *>(to))
        return to_lc->getDictionaryType()->equals(*from);
    return false;
}
void checkVersionColumnTypesConversion(const IDataType * old_type, const IDataType * new_type, const String column_name)