Undo Pass

Single scan backwards in log, undoing actions of ``loser'' transactions
- for each transaction, when a log record is found, use prev_LSN fields to find next record to be undone
- can skip parts of the log with no records from loser transactions
- don't perform any undo for CLRs (note: UndoNxtLSN for CLR indicates next record to be undone, can skip intermediate records of that transactions)

Single scan backwards in log, undoing actions of ``loser'' transactions