RE: Subscription statistics are not dropped at DROP SUBSCRIPTION in some cases

Dear Sawada-san,

Thank you for giving the patch! I confirmed that the problem you raised could be
occurred on the HEAD, and the test you added could reproduce that. When the stats
entry has been removed but pg_stat_get_subscription_stats() is called, the returned
values are set as 0x0.
Additionally, I have checked other pgstat_drop_* functions, and I could not find
any similar problems.

A comment:

```
+ /*
+ * Tell the cumulative stats system that the subscription is getting
+ * dropped.
+ */
+ pgstat_drop_subscription(subid);
```

Isn't it better to write down something you said as comment? Or is it quite trivial?

> There is a chance the
> transaction dropping the subscription fails due to network error etc
> but we don't need to worry about it as reporting the subscription drop
> is transactional.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED