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. - 

" Provided column of type " + column.type->getName() + ".", ErrorCodes::BAD_TYPE_OF_FIELD);
                miss_column = false;
                break;
            }
        }
        if (miss_column)
            throw Exception("Sign column " + sign_column + " does not exist in table declaration.", ErrorCodes::NO_SUCH_COLUMN_IN_TABLE);
    };
    
    auto check_version_column = [this, & columns](bool is_optional, const std::string & storage)
    {
        if (version_column.empty())
        {
            if (is_optional)
                return;
            throw Exception("Logical error: Version column for storage " + storage + " is empty", ErrorCodes::LOGICAL_ERROR);
        }
        bool miss_column = true;
        for (const auto & column : columns)
        {
            if (column.name == version_column)
            {
                if (!column.type->canBeUsedAsVersion())
                    throw Exception("The column " + version_column +
                        " cannot be used as a version column for storage " + storage +
                        " because it is of type " + column.type->getName() +
                        " (must be of an integer type or of type Date/DateTime/DateTime64)", ErrorCodes::BAD_TYPE_OF_FIELD);
                miss_column = false;
                break;
            }