[RFR] Use data provider without saga

[fzaninotto]

• Rename the fetch directory in ra-core to dataProvider
• [BC Break] Add <DataProviderContext> and use it in <AdminCore>
• Copy fetch and undo sagas logic inside useDataProvider
• Bypass these sagas in useDataProvider by no longer including the fetch meta
• [BC Break] Reimplementing the Undo feature using an EventEmitter
• Fix tests
• Add Upgrade guide

Overall this is completely transparent unless you're using a custom app. The benefit is that the useDataProvider code is much easier to follow, with less layers to look at. Also, merging fetch and undo really makes sense as the Undo feature can only undo a fetch action.

The cons are the use of the EventEmitter, which I preferred over a temporary redux state, and the removal of the possibility to hack the react-admin dataProvider logic by replacing the fetch saga.

From that point on, technically we could remove the fetch and auth sagas, as well as the crudXXX action creators. However, we keep them for backward compatibility's sake.

[djhi]

Not used ?

[djhi]

I'm not sure we need this reducer to handle optimistic detection. Can't w achieve the same with metas on the action ?

[fzaninotto]

No, because the use case is the following:

1. user clicks on Delete post
2. the useDataProvider hook optimistically deletes the record from the store and redirects to the post list
3. the list calls the dataProvider for the list of posts on mount
4. While waiting for the result, the list shows the posts from the sorte, where the deleted post has been removed
5. As the deletion wasn't performed on the server yet, the server returns results including the deleted record
6. The list shows the deleted post again

To avoid that, the 'optimistic mode' blocks queries while an optimistic update is pending, so 3 and 5 never happen.