pgsql: Allow NOT VALID foreign key constraints on partitioned tables

Allow NOT VALID foreign key constraints on partitioned tables

This feature was intentionally omitted when FKs were first implemented
for partitioned tables, and had been requested a few times; the
usefulness is clear.

Validation can happen for each partition individually, which is useful
to contain the number of locks held and the duration; or it can be
executed for the partitioning hierarchy as a single command, which
validates all child constraints that haven't been validated already.

This is also useful to implement NOT ENFORCED constraints on top.

Author: Amul Sul <sulamul(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAJ_b96Bp=-ZwihPPtuaNX=SrZ0U6ZsXD3+fgARO0JuKa8v2jQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b663b9436e7509b5e73c8c372539f067cd6e66c1

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 2 -
src/backend/commands/tablecmds.c | 150 +++++++++++++++++++++---------
src/test/regress/expected/foreign_key.out | 97 +++++++++++++++++--
src/test/regress/sql/foreign_key.sql | 72 +++++++++++++-
4 files changed, 263 insertions(+), 58 deletions(-)