Modify `Change.diff()` to include current data revision in each
delta reported back. The current data revision is stored in
`delta.prev`.
Modify `PersistedModel.bulkUpdate()` to check that the current data
revision matches `delta.prev` and report a conflict if a third party
has modified the database under our hands.
Fix `Change` implementation and tests so that they are no longer
attempting to create instances with duplicate ids.
(This used to work because the memory connector was silently
converting such requests to updateOrCreate/findOrCreate.)
87940a4b58