persist: Refactor slightly to look more like v5-using code.

This is a pure refactor that just reorganizes the code slightly.  This
version is perhaps cleaner in itself; but what really motivates the
change is to make it look more like what we'll have for using
redux-persist v5, to shrink the upcoming diff that does that.

Based on @roberthoenig's drafts in #2762 of the redux-persist upgrade.

src/boot/store.js

@@ -1,6 +1,7 @@
 /* @flow */
 import { applyMiddleware, compose, createStore } from 'redux';
 import { persistStore, autoRehydrate } from 'redux-persist';
+import type { Config } from 'redux-persist';
 
 import config from '../config';
 import rootReducer from './reducers';
@@ -17,15 +18,13 @@ import ZulipAsyncStorage from './ZulipAsyncStorage';
 
 const store = compose(applyMiddleware(...middleware), autoRehydrate())(createStore)(rootReducer);
 
+const reduxPersistConfig: Config = {
+  whitelist: [...config.storeKeys, ...config.cacheKeys],
+  // $FlowFixMe: https://github.com/rt2zz/redux-persist/issues/823
+  storage: ZulipAsyncStorage,
+};
+
 export const restore = (onFinished?: () => void) =>
-  persistStore(
-    store,
-    {
-      whitelist: [...config.storeKeys, ...config.cacheKeys],
-      // $FlowFixMe: https://github.com/rt2zz/redux-persist/issues/823
-      storage: ZulipAsyncStorage,
-    },
-    onFinished,
-  );
+  persistStore(store, reduxPersistConfig, onFinished);
 
 export default store;