[9.20] fix: usr: Fix inbound IXFR performance regression.

Very large inbound IXFR transfers were much slower compared to BIND
9.18. The performance was improved by adding specialized logic to
handle IXFR transfers.

Closes #5442

Backport of MR !11077

Merge branch 'backport-5442-ixfr-batch-transaction-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11355