Merge db.Iterate and IterateObjects (#21641)

These two functions are similiar, merge them.

modules/doctor/breaking.go

@@ -18,10 +18,9 @@ import (
 func iterateUserAccounts(ctx context.Context, each func(*user.User) error) error {
 	err := db.Iterate(
 		ctx,
-		new(user.User),
 		builder.Gt{"id": 0},
-		func(idx int, bean interface{}) error {
-			return each(bean.(*user.User))
+		func(ctx context.Context, bean *user.User) error {
+			return each(bean)
 		},
 	)
 	return err