pgsql: Fix WAL format incompatibility introduced by backpatching of 52a

Fix WAL format incompatibility introduced by backpatching of 52ac6cd2d0

52ac6cd2d0 added new field to ginxlogDeletePage and was backpatched to 9.4.
That led to problems when patched postgres instance applies WAL records
generated by non-patched one. WAL records generated by non-patched instance
don't contain new field, which patched one is expecting to see.

Thankfully, we can distinguish patched and non-patched WAL records by their data
size. If we see that WAL record is generated by non-patched instance, we skip
processing of new field. This commit comes with some assertions. In
particular, if it appears that on some platform struct data size didn't change
then static assertion will trigger.

Reported-by: Simon Riggs
Discussion: https://postgr.es/m/CANP8%2Bj%2BK4whxf7ET7%2BgO%2BG-baC3-WxqqH%3DnV4X2CgfEPA3Yu3g%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Simon Riggs, Alvaro Herrera
Backpatch-through: 9.4

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f8a69a68a08715ab5101a9a5eed4e08ba0b3946b

Modified Files
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src/backend/access/gin/ginxlog.c | 19 ++++++++++++++++++-
src/include/access/gin_private.h | 10 ++++++++++
2 files changed, 28 insertions(+), 1 deletion(-)