State persistence (#3716)

Adds a new state persistence mechanisms to all of the navigators via createNavigationContainer

    There are two new props that you can provide to a navigation container: `storageKey` and `renderLoading`.

    `storageKey` is the string key to be used with AsyncStorage while saving and persisting navigation state. To disable persistence, set to null.

    `renderLoading` allows you to render something while the navigator re-hydrates its state and loads its initial URL. By default this returns null, but Expo users will want to render a <AppLoading /> component for smooth app launches

    There is also functionality in this PR to observe errors that come from re-hydrating state, and gracefully recover by dispatching an init action.

    Also this revises the init action to *reset* the navigation state, rather than preserve the previous state.

flow/react-navigation.js

@@ -538,6 +538,8 @@ declare module 'react-navigation' {
       NavigationAction
     ) => void,
     navigation?: NavigationScreenProp<S>,
+    persistenceKey?: ?string,
+    renderLoadingExperimental?: React$ComponentType<{}>,
     screenProps?: *,
     navigationOptions?: O,
   }>;