The logic required to populate the right bits of `RoomData` tends towards
the complete `/sync` response struct, so just use the actual response struct
and save the hassle of mapping between the two. It may not make much difference
in its current form, but the next PR will make use of this.
This PR has no functional changes.
This is only 'mostly' correct currently, because what should be no-op dupe
joins will actually trigger the entire room state to be re-sent.
Bizarrely, it's significantly easier to just do that than work out if we should,
and there are no client-visible effects to doing so, so we just do it for now.
* Split out notifying /sync requests and calculating sync responses
The logic for notifying /sync requests is about to get really
complicated as we optimise when to wake up requests, so split
out that code into a separate struct to isolate it and make
it easier to unit test.
