WIP: ALTER TABLE ... ALTER CONSTRAINT ... SET DEFERRABLE on UNIQUE or PK

Hi all

Attached is a patch to implement ALTER TABLE ... ALTER CONSTRAINT ...
SET DEFERRABLE on UNIQUE or PRIMARY KEY constraints.

Currently only FOREIGN KEY constraints are supported. Others are rejected with:

constraint \"%s\" of relation \"%s\" is not a foreign key constraint

The patch also adds some regression tests for DEFERRABLE constraints.

The ALTER doesn't take effect in the session it's run in, which makes
me suspect I need to do additional cache invalidations - maybe the
index backing the constraint? Anyway, posted here as-is because I'm
out of time for now and it might be useful for someone who's looking
for info on this.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services