pg_dump -j against standbys

pg_dump -j against a standby server returns a pretty bad error message when
pointed at a standby node:

pg_dump: [archiver (db)] query failed: ERROR: cannot assign TransactionIds
during recovery
pg_dump: [archiver (db)] query was: SELECT pg_export_snapshot()

That looks quite scary to the user, and also throws errors in the server
log which monitoring tools or DBAs will react to.

PFA a patch that changes this to:
pg_dump: Synchronized snapshots are not supported on standby servers.
Run with --no-synchronized-snapshots instead if you do not need
synchronized snapshots.

which is a message similar (the hint identical) the one you get if you
point it at a version older than 9.2 which doesn't have sync snapshots.

I think the cleanest way to do it is to just track if it's a standby in the
AH struct as written.

Comments?

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/