This patch updates `ThreadPagination` to hold a `ThreadEventCacheInner`
instead of a `RoomEventCacheInner`! It makes more sense and it
splits/isolates the types even more.
`RoomEventCache::thread_pagination` is now async and returns a
`Result<ThreadPagination>` because it needs to load its state to fetch
a `ThreadEventCache`. Later, accessing a thread wouldn't happen in
`RoomEventCache` but in `Caches`, one step at a time.
Ivan Enderlin
b85bb5463f