Journaling design: group-specific or field-specific

[atbenmurray]

As of the writing of this issue, journaling in ExeTera is on a group level; that is to say there is one set of journaling timestamp fields per group and all fields are journaled in the same way regardless of whether a given field changed or not. This is efficient from the point of view of timestamps but not from the point of view of fields that don't change. Investigate which representation is better and specifically whether it is better to have field-specific journaling or stick with group-level journaling