pgsql: Reduce lock levels of some trigger DDL and add FKs

Reduce lock levels of some trigger DDL and add FKs

Reduce lock levels to ShareRowExclusive for the following SQL
 CREATE TRIGGER (but not DROP or ALTER)
 ALTER TABLE ENABLE TRIGGER
 ALTER TABLE DISABLE TRIGGER
 ALTER TABLE … ADD CONSTRAINT FOREIGN KEY

Original work by Simon Riggs, extracted and refreshed by Andreas Karlsson
New test cases added by Andreas Karlsson
Reviewed by Noah Misch, Andres Freund, Michael Paquier and Simon Riggs

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0ef0396ae1687bf738d4703773d55467c36b2bcd

Modified Files
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doc/src/sgml/mvcc.sgml                        |   12 +-
doc/src/sgml/ref/alter_table.sgml             |    3 +
src/backend/commands/tablecmds.c              |   35 +-
src/backend/commands/trigger.c                |    4 +-
src/test/isolation/expected/alter-table-1.out | 1646 ++++++++++++++++++-------
src/test/isolation/isolation_schedule         |    3 +
src/test/regress/expected/alter_table.out     |   22 +-
7 files changed, 1236 insertions(+), 489 deletions(-)