Add temporal PRIMARY KEY and UNIQUE constraints

Add WITHOUT OVERLAPS clause to PRIMARY KEY and UNIQUE constraints.
These are backed by GiST indexes instead of B-tree indexes, since they
are essentially exclusion constraints with = for the scalar parts of
the key and && for the temporal part.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com

src/backend/commands/trigger.c

@@ -841,6 +841,7 @@ CreateTriggerFiringOn(CreateTrigStmt *stmt, const char *queryString,
 											  true, /* islocal */
 											  0,	/* inhcount */
 											  true, /* noinherit */
+											  false,	/* conwithoutoverlaps */
 											  isInternal);	/* is_internal */
 	}