pgsql: Mark commit and abort WAL records with XLR_SPECIAL_REL_UPDATE.

Mark commit and abort WAL records with XLR_SPECIAL_REL_UPDATE.

If a commit or abort record includes "dropped relfilenodes", then replaying
the record will remove data files. That is surely a "special rel update",
but the records were not marked as such. Fix that, teach pg_rewind to
expect and ignore them, and add a test case to cover it.

It's always been like this, but no backporting for fear of breaking
existing applications. If an application parsed the WAL but was not
handling commit/abort records, it would stop working. That might be a good
thing if it really needed to handle the dropped rels, but it will be caught
when the application is updated to work with PostgreSQL v14 anyway.

Discussion: https://www.postgresql.org/message-id/07b33e2c-46a6-86a1-5f9e-a7da73fddb95%40iki.fi
Reviewed-by: Amit Kapila, Michael Paquier

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a28d731a1187e8d9d8c2b6319375fcbf0a8debd5

Modified Files
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src/backend/access/transam/xact.c |  2 ++
src/bin/pg_rewind/parsexlog.c     | 13 +++++++++++++
src/bin/pg_rewind/t/001_basic.pl  | 15 ++++++++++++++-
3 files changed, 29 insertions(+), 1 deletion(-)