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

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@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/66mqpfyti3qhfttcsv6r2lbvqqd32rrmpn6i47ovrsnvguts46%40gou54xc>
Backpatch through: 17