This is part #1 for of the DEFERRED CONSTRAINT TRIGGER support.

This is part #1 for of the DEFERRED CONSTRAINT TRIGGER support.
Implements the CREATE CONSTRAINT TRIGGER and SET CONSTRAINTS commands.

TODO:
    Generic builtin trigger procedures
    Automatic execution of appropriate CREATE CONSTRAINT... at CREATE TABLE
    Support of new trigger type in pg_dump
    Swapping of huge # of events to disk

Jan