fix: dev: step() could ignore rollbacks

The `step()` function (used for stepping to the prececessor or successor of a database node) could overlook a node if there was an rdataset that was marked IGNORE because it had been rolled back, covering an active rdataset under it.

Closes #5170

Merge branch '5170-step-ignores-rollback' into 'main'

See merge request isc-projects/bind9!10103