Function to get a txn's replication origin

Hi all

During the committs / replorigin split, the function to query the
replication origin of a transaction from user space was lost.

A function to do so is utterly trivial, e.g.

Datum
pg_get_xact_replication_origin(PG_FUNCTION_ARGS)
{
TransactionId xid = PG_GETARG_TRANSACTIONID(0);
TimestampTz ts;
RepOriginId nodeid;

if (TransactionIdGetCommitTsData(xid, &ts, &nodeid))
PG_RETURN_INT32((int32)nodeid);
else
PG_RETURN_NULL();
}

... but it probably makes more sense to extend pg_xact_commit_timestamp
with support for returning the origin too.

pg_xact_commit_timestamp() is a scalar function so I don't want to extend
it directly because that'll upset existing callers.

pg_xact_commit_timestamp_origin()

(unsure if a "with" or "and" is appropriate or too long).

Attached. Also add 'roident' to pg_last_committed_xact() to make the info
available there too. It was already record-returning so I'm not overly
concerned about adding a column.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services