Experiment with descriptors used by SQLAlchemy

[JukkaL]

Similar to #2564 but for SQLAlchemy.

[ilevkivskyi]

I think this issue can be closed now. We played with descriptors for SQLAlchemy for some time and this resulted in several PRs polishing corner cases: overloaded descriptors, union descriptors, __get__ vs __getattr__, descriptor assignment on class vs instance, etc. There may be some patterns needed for Django, but we have a separate issue for this.