Next step towards 64bit XIDs: Switch to FullTransactionId for PGPROC->xid and XLogRecord->xl_xid

Hi!

As were discussed in [0] our overall goal is to make Postgres 64 bit XIDs.
It's obvious, that such a big patch set
couldn't possible to commit "at once". SLUR patch set [1] was committed a
short while ago as a first significant
step in this direction.

This thread is a next step in this enterprise. My objective here is to
commit some changes, which were mandatory,
as far as I understand, for any type of 64 XIDs implementation. And I'm
sure there will be points for discussion here.

My original intention was to make PGPROC->xmin, PGPROC->xid and
PROC_HDR->xids 64bit. But in reality,
it turned out to be much more difficult than I expected. On the one hand,
the patch became too big and on the other
hand, it's heavily relayed on epoch and XID "adjustment" to FXID. Therefore,
for now, I decided to limit myself to
more atomic and independent changes. However, as I said above, these
changes are required for any implementation
of 64bit XIDs.

So, PFA patches to make switch PGPROC->xid and XLogRecord->xl_xid to
FullTransactionId.

As always, any opinions are very welcome!

[0]
https://www.postgresql.org/message-id/CACG=ezZe1NQSCnfHOr78AtAZxJZeCvxrts0ygrxYwe=pyyjVWA@mail.gmail.com
[1]
https://www.postgresql.org/message-id/flat/CAJ7c6TPDOYBYrnCAeyndkBktO0WG2xSdYduTF0nxq%2BvfkmTF5Q%40mail.gmail.com

--
Best regards,
Maxim Orlov.