[9.20] fix: dev: Bound memory use during incoming zone transfers

During an incoming zone transfer, an optimization could let
the batch of pending records grow without bound for a large
zone, raising memory usage. It gave no measurable performance
benefit, so it has been removed.

Closes #5958

Backport of MR !12141

Merge branch 'backport-5958-no-name-boundary-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12142