Optimize write to binlog

The current "legacy" method of writing transactions to binlog involves per-event actions, dealing with changes in the event header, which can be optimized away.

In particular the event checksum can be computed as early as at ctor and the binlog write be done with one cache-to-cache copy.

Expected performance gain may vary, and may need to estimated before taking on implementation, but intuitively it looks to be greater for small event size transactions.