A patch for get origin from commit_ts.

Hello hackers,

I am researching about 'origin' in PostgreSQL, mainly it used in logical
decoding to filter transaction from non-local source. I notice that the
'origin' is stored in commit_ts so that I think we are possible to get 'origin'
of a transaction from commit_ts.

But I can not fond any code to get 'origin' from commit_ts, just like it is
producing data which nobody cares about. Can I know what's the purpose
of the 'origin' in commit_ts? Do you think we should add some support
to the careless data?

For example, I add a function to get 'origin' from commit_ts:
=======================================
postgres=# select pg_xact_commit_origin('490');
pg_xact_commit_origin
-----------------------
test_origin
(1 row)

postgres=# select pg_xact_commit_origin('491');
pg_xact_commit_origin
-----------------------
test_origin1
(1 row)

postgres=#
=======================================

Regards,
Highgo Software (Canada/China/Pakistan)
URL : www.highgo.ca
EMAIL: mailto:movead(dot)li(at)highgo(dot)ca