Change tracking for updates

[bencrouse]

Is your feature request related to a problem? Please describe.
When saving an update to a model, the entire document is overwritten in the database, as opposed to only the fields that have changed, which could open you up to concurrency issues. Tracking changes can also be useful when implementing something like audit log.

Describe the solution you'd like
An update that only $sets the changed fields and a function API (e.g. model.HasChanged("FieldName")) to check what fields have changed in the hooks.

Describe alternatives you've considered
I'm not sure how this could implemented without changes to mgm, but open to ideas for sure!

Additional context
Would you all be open to a PR to add this if I put some work into it?

[mehran-prs]

Hi @bencrouse
Tracking changes in models could be a little be complicated.
I think the more simple solution for concurrency issues could be MongoDB transactions and optimistic locks which will be added after merging PR #56

[bencrouse]

Ok I didn't see that PR, which will certainly solve for the concurrency concern, thank you!