RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear hackers,

Here is a new patch.

Previously I wrote:
> Based on above idea, I made new version patch which some functionalities were
> exported from pg_resetwal. In this approach, pg_upgrade itself removed WALs and
> then create logical slots, then pg_resetwal would be called with new option
> --no-switch, which avoid to switch a WAL segment file. The option is only used
> for the upgrading purpose so it is not written in doc and usage(). This option
> is not required if pg_resetwal -o does not discard WAL records. Please see the
> fork thread [1].

But for now, these changes were reverted because changing pg_resetwal -o stuff
may be a bit risky. This has been located more than ten years so that we should
be more careful for modifying.
Also, I cannot come up with problems if slots are created after the pg_resetwal.
Background processes would not generate decodable changes (listed in [1]), and
BGworkers by extensions could be ignored [2].
Based on the discussion on forked thread [3] and if it is accepted, we will apply
again.

Also. some comments and function name was improved.

[1]: https://www.postgresql.org/message-id/TYAPR01MB58660273EACEFC5BF256B133F50DA%40TYAPR01MB5866.jpnprd01.prod.outlook.com
[2]: https://www.postgresql.org/message-id/CAA4eK1L4JB%2BKH_4EQryDEhyaLBPW6V20LqjdzOxCWyL7rbxqsA%40mail.gmail.com
[3]: https://www.postgresql.org/message-id/flat/CAA4eK1KRyPMiY4fW98qFofsYrPd87Oc83zDNxSeHfTYh_asdBg%40mail.gmail.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED