support a Durable Transaction Log

[hippiehunter]

the remaining task on the path to ACID is a durable transaction log, which is really just a fancy name for an on-disk data structure used as part of a two-phase commit. As transactions are being committed to the underlying ISAM files, Harmony Core would write a log of the operations that it’s performing with information on the state they are in should there be a hardware or software fault that interrupts ISAM operations. In addition to its verification duties, the transaction log would also enable us to undo any changes that were only partially committed.