pgsql: Add various assertions to heap pruning code.

Add various assertions to heap pruning code.

These assertions document (and verify) our high level assumptions about
how pruning can and cannot affect existing items from target heap pages.
For example, one of the new assertions verifies that pruning does not
set a heap-only tuple to LP_DEAD.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=vhvBx1GjF+oueHh8YQcHoQYrMi0F0zFMHEr8yc4sCoA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5cd7eb1f1c32e1b95894f28b277b4e4b89add772

Modified Files
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src/backend/access/heap/pruneheap.c | 84 +++++++++++++++++++++++++++++++++++--
1 file changed, 80 insertions(+), 4 deletions(-)