pgsql: Introduce macros for WAL block reference IDs of some heap record

Introduce macros for WAL block reference IDs of some heap record types

When registering a buffer with the WAL machinery, the caller assigns it a
block reference ID, and replay must read each block back by that same ID.
Today these IDs are bare integers assigned by convention (0, 1, 2, ...),
which is easy to follow when a record registers a single block, or when the
blocks are handled during replay in their registration order.

An upcoming bug fix registers up to two visibility map blocks in addition
to the heap block(s) when clearing the VM, and these are not handled during
replay in a straightforward 1:1, in-registration-order fashion. Relying on
bare integers for the block IDs in that case is error-prone.

Introduce macros naming the block reference IDs for the heap record types
that the upcoming commit extends to register visibility map blocks, so the
registration and replay sites refer to the same block by a meaningful name.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Discussion: https://postgr.es/m/66mqpfyti3qhfttcsv6r2lbvqqd32rrmpn6i47ovrsnvguts46%40gou54xc>
Backpatch through: 17

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7edec8b5784130b9c175cd8f5ab4d029d4502453

Modified Files
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src/backend/access/heap/heapam.c | 97 +++++++++++++++++++++++-----------------
src/include/access/heapam_xlog.h | 28 +++++++++---
2 files changed, 77 insertions(+), 48 deletions(-)