RE: speed up a logical replica setup

Dear Alexander, Euler, Amit,

I also analyzed this failure, let me share it. Here, I think events in below
ordering were occurred.

1. Backend created a publication on $db2,
2. BGWriter generated RUNNING_XACT record, then
3. Backend created a replication slot on $db2.

In this case, the recovery_target_lsn is ahead of the RUNNING_XACT record generated
at step 3. Also, since both bgwriter and slot creation mark the record as
*UNIMPORTANT* one, the writer won't start again even after the
LOG_SNAPSHOT_INTERVAL_MS. The rule is written in BackgroundWriterMain():

```
            /*
             * Only log if enough time has passed and interesting records have
             * been inserted since the last snapshot.  Have to compare with <=
             * instead of < because GetLastImportantRecPtr() points at the
             * start of a record, whereas last_snapshot_lsn points just past
             * the end of the record.
             */
            if (now >= timeout &&
                last_snapshot_lsn <= GetLastImportantRecPtr())
            {
                last_snapshot_lsn = LogStandbySnapshot();
                last_snapshot_ts = now;
            }
```

Therefore, pg_createsubscriber waited until a new record was replicated, but no
activities were recorded, causing a timeout. Since this is a timing issue, Alexander
could reproduce the failure with shorter time duration and parallel running.

IIUC, the root cause is that pg_create_logical_replication_slot() returns a LSN
which is not generated yet. So, I think both mine [1] and Euler's approach [2]
can solve the issue. My proposal was to add an extra WAL record after the final
slot creation, and Euler's one was to use a restart_lsn as the recovery_target_lsn.
In case of primary server, restart_lsn is set to the latest WAL insert position and
then RUNNING_XACT record is generated later.

How do you think?

[1]:
https://www.postgresql.org/message-id/OSBPR01MB25521B15BF950D2523BBE143F5D32@OSBPR01MB2552.jpnprd01.prod.outlook.com
[2]: https://www.postgresql.org/message-id/b1f0f8c7-8f01-4950-af77-339df3dc4684%40app.fastmail.com

Best regards,
Hayato Kuroda
FUJITSU LIMITED