Update bulk export log format to v2

[mikix]

The bulk export log format wiki has been updated to tweak events to make more sense with the upcoming allowPartialManifests feature.

Does this affect our log parsing?

As far as I can tell, our log parsing features are unaffected by the new log format (i.e. we can read either format just fine).

Does this affect our log writing?

Sure, we would need to update our log output to match the new spec. Likely could contain the damage to mainly export_log.py,

For now, we don't yet have plans to pass servers allowPartialManifests , so we don't urgently need to support writing the new log format either.

But it would be a nice to have, to get on board with the latest and greatest format.