magma_stress Compaction not updating UserStats atomically

In magma_stess (a test-only non-shipping program), the compaction transaction to UserStats was patterned after how kv maintains UserStats but that method is broken when there are injected IO Errors. Rather than attempt to maintain UserStats as part of the compaction callback, we need to add a UserStats Write Operation to the end txn that cleans up and removes the dropped collection updates. The fix is trivial and has been well tested.