Improve the efficiency of audit writes for prepare in Region.

At the moment data audit is written to HLOG in the region during prepare (phase 1), followed by a prepare trans state record which is force flushed. This occupies significant time during the prepare. More work is required to determine the best way to optimize this. It may be possible to write data audit at the time of the DML operation (put/delete). Also, we are looking at ways to buffer prepare trans state records to improve throughput.