RE: speed up a logical replica setup

Dear Euler,

I love your proposal, so I want to join the review. Here are my first comments.

01.
Should we restrict that `--subscriber-conninfo` must not have hostname or IP?
We want users to execute pg_subscriber on the target, right?

02.
When the application was executed, many outputs filled my screen. Some of them
were by pg_subscriber, and others were server log. Can we record them into
separated file? I imagined like pg_upgrade.

03.
A replication command is used when replication slots are created. Is there a
reason to use it? I think we do not have to use logical replication walsender mode,
we can use an SQL function instead. pg_create_logical_replication_slot() also outputs
LSN, isn't it sufficient?

04.
As you know, there are several options for publications/subscriptions/replication
slots. Do you have a good way to specify them in your mind?

05.
I found that the connection string for each subscriptions have a setting
"fallback_application_name=pg_subscriber". Can we remove it?

```
postgres=# SELECT subconninfo FROM pg_subscription;
                                   subconninfo
---------------------------------------------------------------------------------
 user=postgres port=5431 fallback_application_name=pg_subscriber dbname=postgres
(1 row)
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED