Save vmbuffer in heap-specific scan descriptors for on-access pruning

Save vmbuffer in heap-specific scan descriptors for on-access pruning

Future commits will use the visibility map in on-access pruning to fix
VM corruption and set the VM if the page is all-visible.

Saving the vmbuffer in the scan descriptor reduces the number of times
it would need to be pinned and unpinned, making the overhead of doing so
negligible.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/C3AB3F5B-626E-4AAA-9529-23E9A20C727F%40gmail.com