Fix bthread_usleep return unexpected error

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

#2435 的改动可能会导致bthread_usleep实际上成功执行，但是返回ESTOP失败。

_add_sleep_event函数中，在定时器添加成功后，e.meta->current_sleep = sleep_id之前，定时器执行，唤醒了协程。

brpc/src/bthread/task_group.cpp

Lines 757 to 775 in 09141d4

	sleep_id = get_global_timer_thread()->schedule(
	ready_to_run_from_timer_thread, void_args,
	butil::microseconds_from_now(e.timeout_us));
	if (!sleep_id) {
	// fail to schedule timer, go back to previous thread.
	g->ready_to_run(e.tid);
	return;
	}
	// Set TaskMeta::current_sleep which is for interruption.
	const uint32_t given_ver = get_version(e.tid);
	{
	BAIDU_SCOPED_LOCK(e.meta->version_lock);
	if (given_ver == *e.meta->version_butex && !e.meta->interrupted) {
	e.meta->current_sleep = sleep_id;
	return;
	}
	}

协程被唤醒后，current_sleep为0、interrupted为false，返回了ESTOP错误。

brpc/src/bthread/task_group.cpp

Lines 804 to 808 in 09141d4

	if (e.meta->current_sleep == 0 && !e.meta->interrupted) {
	// Fail to `_add_sleep_event'.
	errno = ESTOP;
	return -1;
	}

What is changed and the side effects?

Changed:

使用一个特定的成员变量代替current_sleep==0来表示定时器添加失败。

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

---

Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[chenBright]

@wwbmmm 有空看看这个问题

[wwbmmm]

LGTM