To handle cancellation, we ensure that `after_callback`s and
`exception_callback`s are always run, since the transaction will
complete on another thread regardless of cancellation.
We also wait until everything is done before releasing the
`CancelledError`, so that logging contexts won't get used after they
have been finished.
Signed-off-by: Sean Quah <seanq@element.io>
Sean Quah