Filter initial sync to handle large rooms (eg Matrix HQ)

Some Matrix users will log into Circles on their main account. Some of these users will be joined to very large rooms like Matrix HQ #matrix:matrix.org, which has around 60k members. Without some careful handling, these rooms will present a challenge.

In Circles Android, the fix that enabled us to handle Matrix HQ was to set a sync filter for the intial sync, to:

1. Enable lazy-loading room members (lazy_load_members)
2. Enable per-thread notification counts (unread_thread_notifications)

Now we need to do the same thing for iOS.