Fix vat snapshots

I discovered that my snapshotting code wasn't doing the right thing within churns. Every event within a churn was being given the exact same snapshot, but really each event needs to get a snapshot of the transactormap at the time the event happened. In order to do this, I added a transactormap-reparent procedure that creates a new transactormap with a new parent. By reparenting the transactormap onto a copy of the vat whactormap and calling transactormap-merge!, we get the desired snapshot. In the future we'll remove the need for this dance by storing only the actormap deltas, but for now this makes things correct with full snapshots.