pgsql: Ensure TOAST storage parameters have unsettable defaults.

Ensure TOAST storage parameters have unsettable defaults.

A follow-up commit will teach autovacuum and manual VACUUM to fill
in a TOAST table's unset storage parameters from its main table's.
To be able to determine whether a parameter is set, it must default
to a value that users cannot set. Of all the parameters that can
be set on TOAST tables, log_autovacuum_min_duration is the only one
that violates this rule. This commit changes its default to -2 so
that we can tell whether it is set. This shouldn't produce any
user-visible behavior changes.

While at it, add assertions to ensure all storage parameters that
can be set on TOAST tables continue to follow this rule.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Greg Burd <greg(at)burd(dot)me>
Tested-by: solai v <solai(dot)cdac(at)gmail(dot)com>
Discussion: https://postgr.es/m/aFRxC1W_kZU9OjJ9%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/63979e5bea14c533dadc8f44cee0ec7a2899ccf5

Modified Files
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src/backend/access/common/reloptions.c | 76 +++++++++++++++++++++++++++++++++-
src/backend/postmaster/autovacuum.c | 2 +-
2 files changed, 76 insertions(+), 2 deletions(-)