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

Dear hackers,

My PoC does not read and copy logical mappings files to new node, but I
did not analyzed in detail whether it is correct. Now I have done this and
considered that they do not have to be copied because transactions which executed
at the same time as rewriting are no longer decoded. How do you think?
Followings my analysis.

## What is logical mappings files?

Logical mappings file is used to track the system catalogs while logical decoding
even if its heap file is written. Sometimes catalog heaps files are modified, or
completely replaced to new files via VACUUM FULL or CLUSTER, but reorder buffer
cannot not track new one as-is. Mappings files allow to do them.

The file contains key-value relations for old-to-new tuples. Also, the name of
files contains the LSN where the triggered event is happen.

Mappings files are needed when transactions which modify catalogs are decoded.
If the LSN of files are older than restart_lsn, they are no longer needed then
removed. Please see CheckPointLogicalRewriteHeap().

## Is it needed?

I think this is not needed.
Currently pg_upgrade dumps important information from old publisher and then
execute pg_create_logical_replication_slot() on new one. Apart from
pg_copy_logical_replication_slot(), retart_lsn and confirmed_flush_lsn for old
slot is not taken over to the new slot. They are recalculated on new node while
creating. This means that transactions which have modified catalog heaps on the
old publisher are no longer decoded on new publisher.

Therefore, the mappings files on old publisher are not needed for new one.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED