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/backend/commands/sequence.c

@@ -1602,3 +1602,19 @@ seq_redo(XLogRecPtr lsn, XLogRecord *record)
 
 	pfree(localpage);
 }
+
+/*
+ * Flush cached sequence information.
+ */
+void
+ResetSequenceCaches(void)
+{
+	SeqTableData *next;
+
+	while (seqtab != NULL)
+	{
+		next = seqtab->next;
+		free(seqtab);
+		seqtab = seqtab->next;
+	}
+}