[Async DB] Modularize Zotero.DataObject.save()

At this point, I just mostly want to check if this is an OK direction.

What this allows us to do is, after extending, say, a `Zotero.Item` class, to override the `_saveData` method in order to write additional data before the DB transaction is committed. It also removes a bunch of boilerplate code from `DataObject` child classes (there's probably more that can be removed)
Yes, I think we need to rename that, since it does return a _different_ class from the argument. "bucket" was the closest I could think of to describe the relationship.