Summary:
Creates a `RootTag` type and refactors the `RootTagContext` module a bit.
This creates space for eventually changing `RootTag` into an opaque type that is only created once by `AppContainer`, and only consumed by native abstractions.
Changelog:
[Internal]
(Note: this ignores all push blocking failures!)
Reviewed By: cpojer
Differential Revision: D21127173
fbshipit-source-id: 60177a6e5e02d6308e87f76d12a271114f8f8fe0
Tim Yung