Move VM update code from lazy_scan_heap() to lazy_scan_prune().

Move VM update code from lazy_scan_heap() to lazy_scan_prune().

Most of the output parameters of lazy_scan_prune() were being
used to update the VM in lazy_scan_heap(). Moving that code into
lazy_scan_prune() simplifies lazy_scan_heap() and requires less
communication between the two.

This change permits some further code simplification, but that
is left for a separate commit.

Melanie Plageman, reviewed by me.

Discussion: http://postgr.es/m/CAAKRu_aM=OL85AOr-80wBsCr=vLVzhnaavqkVPRkFBtD0zsuLQ@mail.gmail.com