Re: partitioned tables referenced by FKs

Here's another rebase. The code is structured somewhat differently from
the previous one, mostly because of the backpatched bugfixes, but also
because of the changes in dependency handling.

I think there's also at least one more bugfix to backpatch (included in
0003 here), related to whether foreign tables are allowed as having FKs,
but AFAICS it just cosmetic: you get an error if you have a foreign
partition and add a FK, but it says "you cannot have constraint
triggers" rather than "FKs are not supported", so it's a bit odd.
0003 also includes Amit L's patch to remove the parentConstraint
argument from ATExecAddForeignKey, which I suppose I should commit
separately.

Odd bugs fixed: a) handle the case where the default partition is the
only one and it's being detached or dropped. b) when a partition was
dropped/detached from the referenced side, the child constraint was left
in place.

I have not yet fixed the "#if 0" section.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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