largely stop using `RoomCreateEventContent`

This became a problem because #foundation-office:matrix.org has a malformed create event with its predecessor set to a string instead of a map.

The solution to this is, unfortunately, to do more shotgun parsing to extract only the desired fields rather than trying to parse the entire content every time. To prevent this kind of problem from happening again, RoomCreateEventContent must only be used for creating new PDUs, existing PDUs must be shotgun-parsed.