createPersistor: Separate serializing from writing.

One thing that should help mitigate zulip#4841 is to narrow the window
during which we're propagating a Redux state change to persistent
storage. This does quite a bit of that, by not having the `setItem`
calls separated by (a) the CPU time to do the serialization, or (b)
yields, as Greg points out [1].

[1] zulip#4694 (comment)

src/third/redux-persist/createPersistor.js

@@ -49,13 +49,18 @@ export default function createPersistor (store, config) {
         updatedSubstates.push([key, state[key]])
       });
 
+      const writes = []
       while (updatedSubstates.length > 0) {
         await new Promise(r => setTimeout(r, 0));
 
         const [key, substate] = updatedSubstates.shift()
-        const storageKey = createStorageKey(key)
-        storage.setItem(storageKey, serializer(substate)).catch(warnIfSetError(key))
+        writes.push([key, serializer(substate)])
       }
+
+      writes.forEach(([key, serializedSubstate]) => {
+        storage.setItem(createStorageKey(key), serializedSubstate).catch(warnIfSetError(key))
+      })
+
       writeInProgress = false
       lastState = state
     })()