RE: issue with synchronized_standby_slots

Dear Shlok,

```
-- Can set synchronized_standby_slots to a non-existent slot name.
ALTER SYSTEM SET synchronized_standby_slots='missing';
SELECT pg_reload_conf();

-- Reset the GUC.
ALTER SYSTEM RESET synchronized_standby_slots;
SELECT pg_reload_conf();
```

pg_reload_conf() is called twice here but I'm not sure it is really needed.
ALTER SYSTEM itself can validate parameters via parse_and_validate_value(elevel=ERROR),
and pg_reload_conf() won't affect synchronously. Also, the backend won't output
even if the parameter is invalid.

Best regards,
Hayato Kuroda
FUJITSU LIMITED