[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)
This doesn't need to be set on all the objects. It can be retrieved from the object type using ```Zotero.DataObjectUtilities.getClassForObjectType(objectType)```. (I'm open to renaming that, though I'm not sure "BucketClass" is clearer.)