RE: Log a warning in pg_createsubscriber for max_slot_wal_keep_size

Dear Shubham,

Thanks for creating a patch. I have one comment about it.

check_publisher() assumed that the SQL function `pg_catalog.current_setting('max_slot_wal_keep_size')`
will return the numeric, but it just return the text representation. I.e., if the parameter is
set to 10MB, the function returns like below:

```
postgres=# SELECT * FROM pg_catalog.current_setting('max_slot_wal_keep_size');
current_setting
-----------------
10MB
(1 row)
```

Your patch can work well because atoi() ignores the latter part of the string,
e.g., "10MB" is converted to "10", but this is not clean. I suggest either of
1) accepting the value as the string, or 2) using an SQL function pg_size_bytes()
to get max_slot_wal_keep_size.

Best regards,
Hayato Kuroda
FUJITSU LIMITED