pgsql: Save vmbuffer in heap-specific scan descriptors for on-access pr

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/99bf1f8aa6cd8029cb9ce17d4a804eed386bec97

Modified Files
--------------
src/backend/access/heap/heapam.c         | 12 +++++++++++-
src/backend/access/heap/heapam_handler.c | 12 ++++++++++--
src/backend/access/heap/pruneheap.c      |  6 +++++-
src/include/access/heapam.h              | 19 ++++++++++++++++---
4 files changed, 42 insertions(+), 7 deletions(-)