RE: Follow-up review items for update_deleted

[Hayato Kuroda (Fujitsu)]

Dear Hou,

Thanks for the patch. I reviewed 0001 and could not find critical issues.
Few comments;

01.
```
                                        /* Remember the retained databases for 
the next cycle. */
                                        current_dbids = 
lappend_oid(current_dbids, sub->dbid);
```

list_append_unique_oid() is usable here.

02. init_conflict_slot_xmin()

Not sure the function name is good because it can be called many times.
How about: reset_conflict_slot_xmin_to_safe_horizon()? Code comment should also
be updated.

03.
```
        /* Write this slot to disk */
        ReplicationSlotMarkDirty();
        ReplicationSlotSave();
```

IIUC they are needed only if the slot is updated.

Best regards,
Hayato Kuroda
FUJITSU LIMITED