pgsql: Check for pending trigger events on far end when dropping an FK

Check for pending trigger events on far end when dropping an FK constraint.

When dropping a foreign key constraint with ALTER TABLE DROP CONSTRAINT,
we refuse the drop if there are any pending trigger events on the named
table; this ensures that we won't remove the pg_trigger row that will be
consulted by those events.  But we should make the same check for the
referenced relation, else we might remove a due-to-be-referenced pg_trigger
row for that relation too, resulting in "could not find trigger NNN" or
"relation NNN has no triggers" errors at commit.  Per bug #14431 from
Benjie Gillam.  Back-patch to all supported branches.

Report: <20161124114911.6530.31200@wrigleys.postgresql.org>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6cbe84c826b51a159825e9843184c7b4a740e4ae

Modified Files
--------------
src/backend/commands/tablecmds.c          | 18 ++++++++++++++++++
src/test/regress/expected/foreign_key.out | 13 +++++++++++++
src/test/regress/sql/foreign_key.sql      | 13 +++++++++++++
3 files changed, 44 insertions(+)