Summary:
Makes the delta bundle data structures more consistent.
The changes are as follows:
* There are now two types of JSON bundles that can be downloaded from the delta endpoint. Base bundles (`Bundle` type), and Delta bundles (`DeltaBundle` type).
* The `reset` boolean is renamed to `base`.
* `pre` and `post` properties are now strings.
* Only `Bundle` can define `pre` and `post` properties.
* The `delta` property is renamed to `modules`.
* Deleted modules are now listed inside of the `deleted` property, which is only defined by `DeltaBundle`.
Reviewed By: mjesun
Differential Revision: D10446831
fbshipit-source-id: 40e229a2811d48950f0bad8dd341ece189089e9b
1eedf05651