Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

在单task中投递3次swoole_timer_after定时器，执行顺序，（1）执行15秒，（2）执行2秒，（3）当2秒定时器执行后，在投递一次2秒的定时器

2. What did you expect to see?

第三次定时器，在2秒的时候被执行

3. What did you see instead?

第三次的定时器，没有在2秒的时候执行，而是根据15秒的定时器剩余多少时间执行被推迟

4. What version of Swoole are you using (php --ri swoole)?

4.0.2

5. What is your machine environment used (including version of kernel & php & gcc) ?

Linux version 2.6.32-696.6.3.el6.x86_64 (mockbuild@c1bl.rdu2.centos.org) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-18) (GCC) )

PHP 7.1.18 (cli) (built: Jun 2 2018 07:10:30) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies

6. If you are using ssl, what is your openssl version?

N/A