pgsql: Create action triggers when partitions are detached

Create action triggers when partitions are detached

Detaching a partition from a partitioned table that's constrained by
foreign keys requires additional action triggers on the referenced side;
otherwise, DELETE/UPDATE actions there fail to notice rows in the table
that was partition, and so are incorrectly allowed through.  With this
commit, those triggers are now created.  Conversely, when a table that
has a foreign key is attached as a partition to a table that also has
the same foreign key, those action triggers are no longer needed, so we
remove them.

Add a minimal test case verifying (part of) this.

Authors: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/f2b8ead5-4131-d5a8-8016-2ea0a31250af@lab.ntt.co.jp

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/123cc697a8eb0827e82ceea3f6da55b2f05eb422

Modified Files
--------------
src/backend/commands/tablecmds.c          | 71 ++++++++++++++++++++++++++++++-
src/test/regress/expected/foreign_key.out | 28 +++++++++++-
src/test/regress/sql/foreign_key.sql      | 19 ++++++++-
3 files changed, 113 insertions(+), 5 deletions(-)