Conduit did not send the event in /sync because of a race condition.
There is a brief moment in time where Conduit accepted the event, but
did not store it yet. So when the client /syncs it updates the since
token without sending the event. I hope I fixed it by significantly
shortening the race-condition period.

json_body is used in places that need authentication. In
case an unknown field is set, Ruma doesn't parse the field
and so doesn't give an error on invalid UTF-8. But Conduit
has parsed and on error makes json_body None. Return an
error to the client instead of generating an internal error.