pgsql: Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY and
HEAP_KEYS_UPDATED cannot appear together, on the faulty assumption that
the latter necessarily referred to an update and not a tuple lock; but
that's wrong, because SELECT FOR UPDATE can use precisely that
combination, as evidenced by the amcheck test case added here.

Remove the Assert(), and also patch amcheck's verify_heapam.c to not
complain if the combination is found. Also, out of overabundance of
caution, update (across all branches) README.tuplock to be more explicit
about this.

Author: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Reviewed-by: Mahendra Singh Thalor <mahi6run(at)gmail(dot)com>
Reviewed-by: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Discussion: https://postgr.es/m/20210124061758.GA11756@nol

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0ccebe779dc0ed1c5f37370f2d5bc59779cf9073

Modified Files
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src/backend/access/heap/README.tuplock | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)