new: usr: implement the 'request-ixfr-max-diffs' configuration option

The new 'request-ixfr-max-diffs' configuration option sets the
maximum number of incoming incremental zone transfer (IXFR) differences,
exceeding which triggers a full zone transfer (AXFR).

Closes #4389

Merge branch '4389-request-ixfr-max-diffs' into 'main'

Closes #4389

See merge request isc-projects/bind9!9094