Add DISCARD SEQUENCES command.

DISCARD ALL will now discard cached sequence information, as well.

Fabrízio de Royes Mello, reviewed by Zoltán Böszörményi, with some
further tweaks by me.

src/test/regress/sql/sequence.sql

@@ -72,6 +72,8 @@ SELECT setval('sequence_test'::regclass, 32);
 SELECT nextval('sequence_test'::text);
 SELECT setval('sequence_test'::regclass, 99, false);
 SELECT nextval('sequence_test'::text);
+DISCARD SEQUENCES;
+SELECT currval('sequence_test'::regclass);
 
 DROP SEQUENCE sequence_test;