Cache initial sync response until it is fully handled

[manuroe]

Initial syncs as we have currently (Sync v2) are expensive for homeservers and clients. With a big account (> 3000 rooms), we can get the response from the server after 70 seconds or more (and if it does not timeout). Then, the iOS app takes 60 seconds to complete the storage to its cache (MXFileStore).

The app is usable before but if the user backgrounds the app too early, it is likely to be killed before cache completion. This means the app will issue another initial /sync request at the next startup.

We should cache the initial sync raw response until the cache is complete. If the app is killed before the cache completion, we will be able to replay this cached initial sync response, saving those 70 seconds of more.