partitioned tables referenced by FKs

Here's a patch to allow partitioned tables to be referenced by foreign
keys.  Current state is WIP, but everything should work; see below for
the expected exception.

The design is very simple: have one pg_constraint row for each partition
on each side, each row pointing to the topmost table on the other side;
triggers appear on each leaf partition (and naturally they don't appear
on any intermediate partitioned table).

There are tests that should cover all the basic features.  pg_upgrade
tests work (containing relevant tables, as regress/foreign_key.sql
leaves them behind for this reason: partitioned-references-partitioned).

There is one requisite feature still missing from this patch: when a
partition on the referenced side is detached or dropped, we must ensure
no referencing row remains on the other side.  For this, I have an
(unmerged and thus unsubmitted here) new ri_triggers.c routine
RI_Inverted_Initial_Check (need to come up with better name, heh) that
verifies this, invoked at DETACH/DROP time.

Also, some general code cleanup and documentation patching is needed.

I'm posting this now so that I can take my hands off it for a few days;
will return to post an updated version at some point before next
commitfest.  I wanted to have this ready for this commitfest, but RL
dictated otherwise.  This patch took a *very long time* to write ... I
wrote three different recursion models for this.


One thing I realized while writing this, is that my commit
3de241dba86f ("Foreign keys on partitioned tables") put function
CloneForeignKeyConstraints() in catalog/pg_constraint.c that should
really have been in tablecmds.c.  In this patch I produced some siblings
of that function still in pg_constraint.c, but I intend to move the
whole lot to tablecmds.c before commit.

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