Add optimized update_or_create Manager method

[tim-schilling]

Problem Statement
Model.objects.update_or_create will always result in a .save() call which means there will always be a new historical record created regardless if there were any changes.

Describe the solution you'd like
There could be a method similar to update_or_create that inspects the updating defaults for changes from the fetched instance. Only if there are actual changes would it make a call to .save().

I have a proposed method here: https://gist.github.com/tim-schilling/b04846542927f6e6be777612e4b89d57

Describe alternatives you've considered
None

Additional context
This would not be compatible with someone who is using DateTimeField(auto_now=True) as a heartbeat field.