pgsql: Allow on-access pruning to set pages all-visible

Allow on-access pruning to set pages all-visible

Many queries do not modify the underlying relation. For such queries, if
on-access pruning occurs during the scan, we can check whether the page
has become all-visible and update the visibility map accordingly.
Previously, only vacuum and COPY FREEZE marked pages as all-visible or
all-frozen.

This commit implements on-access VM setting for sequential scans, tid
range scans, sample scans, bitmap heap scans, and the underlying heap
relation in index scans.

Setting the visibility map on-access can avoid write amplification
caused by vacuum later needing to set the page all-visible, which could
trigger a write and potentially an FPI. It also allows more frequent
index-only scans, since they require pages to be marked all-visible in
the VM.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b46e1e54d078def33b840ae1fa6c5236a7b12ec2

Modified Files
--------------
src/backend/access/heap/heapam.c | 3 +-
src/backend/access/heap/heapam_handler.c | 6 ++-
src/backend/access/heap/pruneheap.c | 66 ++++++++++++++++++++++++--------
src/backend/access/heap/vacuumlazy.c | 2 +-
src/include/access/heapam.h | 3 +-
5 files changed, 59 insertions(+), 21 deletions(-)