pgsql: amcheck: Distinguish interrupted page deletion from corruption.

amcheck: Distinguish interrupted page deletion from corruption.

This prevents false-positive reports about "the first child of leftmost
target page is not leftmost of its level", "block %u is not leftmost"
and "left link/right link pair".  They appeared if amcheck ran before
VACUUM cleaned things, after a cluster exited recovery between the
first-stage and second-stage WAL records of a deletion.  Back-patch to
v11 (all supported versions).

Reviewed by Peter Geoghegan.

Discussion: https://postgr.es/m/20231005025232.c7.nmisch@google.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3c6a05b805c750771dc6d3471ca32b27eb25b33e

Modified Files
--------------
contrib/amcheck/Makefile        |  1 +
contrib/amcheck/meson.build     |  1 +
contrib/amcheck/t/005_pitr.pl   | 82 ++++++++++++++++++++++++++++++++++++++++
contrib/amcheck/verify_nbtree.c | 83 +++++++++++++++++++++++++++++++++++++++--
4 files changed, 163 insertions(+), 4 deletions(-)