c.f. #12993 (comment), point 3
This stores all device list updates that we receive while partial joins are ongoing, and processes them once we have the full state.
Note: We don't actually process the device lists in the same ways as if we weren't partially joined. Instead of updating the device list remote cache, we simply notify local users that a change in the remote user's devices has happened. I think this is safe as if the local user requests the keys for the remote user and we don't have them we'll simply fetch them as normal.
