PT-1919 drop-swap can drop triggers (#483)

* PT-1919 drop-swap can drop triggers

* PT-1919 Update changelog

t/pt-online-schema-change/samples/pt-1919.sql

@@ -0,0 +1,39 @@
+DROP DATABASE IF EXISTS test;
+CREATE DATABASE test;
+USE test;
+
+CREATE TABLE aaa (
+  id int NOT NULL AUTO_INCREMENT,
+  a int DEFAULT 0,
+  b int DEFAULT 0,
+  x int DEFAULT 0,
+  z int DEFAULT NULL,
+  PRIMARY KEY (`id`)
+);
+
+CREATE TABLE bbb (
+  id int NOT NULL AUTO_INCREMENT,
+  aaa_id int DEFAULT NULL,
+  c int DEFAULT 0,
+  d int DEFAULT 0,
+  x int DEFAULT 0,
+  modification_date datetime(3),
+  PRIMARY KEY (`id`),
+  CONSTRAINT `FK_aaa_id` FOREIGN KEY (`aaa_id`) REFERENCES aaa (`id`) 
+);
+
+CREATE TRIGGER before_aaa_upd1 BEFORE UPDATE ON aaa FOR EACH ROW set new.a = old.a+1;
+CREATE TRIGGER before_aaa_upd2 BEFORE UPDATE ON aaa FOR EACH ROW set new.b = old.b+1;
+CREATE TRIGGER after_aaa_upd3 AFTER UPDATE ON aaa FOR EACH ROW update bbb set c = c+1 WHERE aaa_id = NEW.id;
+CREATE TRIGGER after_aaa_upd4 AFTER UPDATE ON aaa FOR EACH ROW update bbb set d = d+1 WHERE aaa_id = NEW.id;
+
+DELIMITER $$
+CREATE TRIGGER before_aaa_upd5 BEFORE UPDATE ON aaa FOR EACH ROW 	BEGIN UPDATE bbb SET modification_date = UTC_TIMESTAMP(3) WHERE aaa_id = NEW.id; END;
+$$
+CREATE TRIGGER after_aaa_upd6 AFTER UPDATE ON aaa FOR EACH ROW 	BEGIN UPDATE bbb SET x = x+1 WHERE aaa_id = NEW.id; END;
+$$
+DELIMITER ;
+
+INSERT INTO aaa (x) VALUES (10),(20),(30),(40),(50);
+INSERT INTO bbb (aaa_id) VALUES (1), (2), (3), (3);
+-- UPDATE aaa SET z=id-1 WHERE id=2;