Remove name boundary optimization

Remove name boundary optimization

In MR !9740, we introduced an optimization that reduces memory usage
by processing rdatas in batches during AXFR.

The maximum batch size is 128, but the batch size was allowed to grow
beyond that limit if all rdatas in a batch were for the same name, as
that allows a more efficient optimization.

This optimization could theoretically allow the batch size arbitrarily
for a sufficient large zone transfer. Since synthetic tests don't show
any performance improvement from the optimization, this MR removes it.