Code review comment for lp:~stub/launchpad/replication

Fix staging & dogfood rebuilds again. Now tested with dumps containing genuine Slony-I cruft.

Also made repair-restored-db.py a little more robust (I was wondering if the Slony UNINSTALL NODE command is just too fragile to bother with, but decided to keep trying to use it anyway as it has likely been improved in more modern versions of Slony-I and will succeed more often once the authdb replication set is somewhere else).