Prototype/example of an adapter that doesn't leverage the internal datastore

[dgarros]

This PR is a prototype to explore if it's possible to build an adapter that is not leveraging the internal datastore (related to #57)

For this example I modified example3 (still in review #53) to bypass the internal datastore by reimplementing the 5 main methods on the adapter that are interacting with the datastore (get / get_all / get_by_uids / add / remove).
I moved the logic of get / get_all and get_by_uids to each model class instead.

it's working but it's significantly slower, especially when doing a sync with a very small diff, I need to investigate further why but it's a good start.
One thing I noticed is that DiffSync is currently expecting get_all and get_by_uids to return a list where it would be useful to accept of generator instead to improve the processing and the memory consumption.

I'd love to get some feedback on this experiment and how we could package that into DiffSync if we want to move forward.

[glennmatthews]

Interesting design! Definitely worth exploring further. What issues did you run into when trying to return a generator rather than a list from get_all and get_by_uids? Have you done any sort of memory profiling to see what sort of difference is observed?

[chadell]

superseded by #106