Remove spurious @preserve_fn decorators

Remove `@preserve_fn` decorators on `on_new_room_event`,
`_notify_pending_new_room_events`, `_on_new_room_event`, `on_new_event`, and
`on_new_replication_data` - none of these functions return a deferred, and the
decorator does nothing unless the wrapped function returns a deferred, so the
decorator was a no-op.