Optimistic UI update: reimplement

[atdiar]

The current implementation of optimistic update is flawed. What needs to happen when using SetDataSetUIOptimistically is:

• do not change the data property yet
• add the mutation to the list of optimistic updates for the *Element
• set the User Interface (the optimistic part)
• on validation or invalidation, the mutation from the optimistic update list is flagged as such
• because when flagging the validation status, we also check whether all mutations have been validated, if they have been, we get the data value which is still unchanged and we apply the changes, then update by using SetDataSetUI.

Also probably want to update a list of invalidations so that the User can be notified of action results.

[atdiar]

Eventually, left it as is for the most part. No clobbering since a request to an endpoint cannot race with itself.

Also, the user should simply not optimistically update interdependent data. It should be hard to do anyway, interdeoendent property are seldom if ever used especially wrt reactivity.