fn set_physical_compaction(&mut self, frontier: AntichainRef<Tr::Time>) {
        self.wrapper.borrow_mut().adjust_physical_compaction(self.physical_compaction.borrow(), frontier);
        self.physical_compaction = frontier.to_owned();
    }
Allows the trace to compact batches of times before `frontier`.
differential-dataflow/src/trace/wrappers/rc.rs