Split DataSource & DataSourceView

Summary:
This diff is primarily cosmetic, just pushing code around to make the API more intuitive. Most importantly, DataSource was split into DataSource and DataSourceView classes, the latter being accessible through `datasource.view`.

The benefit of this is two fold:
1. Conceptually it is much clearer now which operations operate on the _source_ records, and which ones on the derived _view_.
2. This will make it easier in the future to support multiple views to be based on a single data source.

This refactoring also nicely found 2 cases where datasource logic and view logic were mixed.

The only semantic change in this diff is that both DataSource and DataSourceView are now iterable, so that one can do a `for (const record of ds)` / `for (const record of ds.view)`

Reviewed By: nikoant

Differential Revision: D26976838

fbshipit-source-id: 3726e92b3c6ee3417dc66cbbe6e288797eecf70e

desktop/flipper-plugin/src/__tests__/test-utils.node.tsx

@@ -577,7 +577,7 @@ test('plugins can serialize dataSources', () => {
     },
   );
 
-  expect(instance.ds.records).toEqual([4, 5]);
+  expect(instance.ds.records()).toEqual([4, 5]);
   instance.ds.shift(1);
   instance.ds.append(6);
   expect(exportState()).toEqual({