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

Dear hackers,

> > > It was primarily for upgrade purposes only. So, as we can't see a good reason
> to
> > > go via pg_dump let's do it in upgrade unless someone thinks otherwise.
> >
> > Removed the new option in pg_dump and modified the pg_upgrade
> > directly use the slot info to restore the slot in new cluster.
> 
> In this version, creations of logical slots are serialized, whereas old ones were
> parallelised per db. Do you it should be parallelized again? I have tested locally
> and felt harmless. Also, this approch allows to log the executed SQLs.

I updated the patch to allow parallel executions. Workers are launched per slots,
each one connects to the new node via psql and executes pg_create_logical_replication_slot().
Moreover, following points were changed for 0002.

* Ensured to log executed SQLs for creating slots.
* Fixed an issue that 'unreserved' slots could not be upgrade. This change was 
  not expected one. Related discussion was [1].
* Added checks for output plugin libraries. pg_upgrade ensures that plugins
  referred by old slots were installed to the new executable directory. 

[1]: https://www.postgresql.org/message-id/TYAPR01MB5866FD3F7992A46D0457F0E6F50BA%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED