RE: BUG #18988: DROP SUBSCRIPTION locks not-yet-accessed database

Dear Vignesh, Dilip,

I found another corner case:

```
postgres=# CREATE SUBSCRIPTION sub CONNECTION 'dbname=db1 user=postgres port=5431' PUBLICATION pub1 WITH
(connect=false,slot_name=sub);
 
WARNING:  subscription was created, but is not connected
HINT:  To initiate replication, you must manually create the replication slot, enable the subscription, and refresh the
subscription.
CREATE SUBSCRIPTION
postgres=# DROP SUBSCRIPTION sub ;
... (won't return)
```

Because still can explicitly specify the slot_name while creating the subscription.
Another pattern is to run ALTER SUBSCRIPTION SET (slot_name) command after the
CREATE SUBSCRIPTION WITH (connect=false);.

Should we fix the case? If so, how?

Best regards,
Hayato Kuroda
FUJITSU LIMITED