Renaming of pg_xlog and pg_clog

Hi all,

I am relaunching $subject as 10 development will begin soon. As far as
I know, there is agreement that we can do something here. Among the
different proposals I have found:
- pg_clog renamed to pg_commit_status, pg_xact or pg_commit
- pg_xlog renamed to pg_xjournal, pg_wal or pg_journal

Another idea from Stephen
(https://www.postgresql.org/message-id/20160826003715.GG4028@tamriel.snowman.net)
would be to put everything that is temporary and not WAL-logged into a
single place to facilitate the filtering work of backup tools.

A straight renaming would be a simple patch (including pg_upgrade
part), and if we actually do it for 10.0 it would be good to do it now
instead of in 3 months. I don't mind writing a patch for it.

Now, one of the things discussed as well was that we may want to still
keep pg_xlog, and soft-link to pg_journal or whatever-the-new-name is
to not break the existing tools. Personally, I'd prefer a hard break.
That would not be complicated to fix for any tool maintainers.

Thoughts?
-- 
Michael