There's no guarantee that module callbacks will handle cancellation
appropriately. Protect module callbacks with read semantics from
cancellation and avoid swallowing `CancelledError`s that arise.
Other module callbacks, such as the `on_*` callbacks, are presumed to
live on code paths that involve writes and aren't cancellation-friendly.
These module callbacks have been left alone.
Signed-off-by: Sean Quah <seanq@element.io>
