`delay_cancellation` behaves like `stop_cancellation`, except it
delays `CancelledError`s until the original `Deferred` resolves.
This is handy for unifying cleanup paths and ensuring that uncancelled
coroutines don't use finished logcontexts.

Signed-off-by: Sean Quah <seanq@element.io>

`@cached` and `@cachedList` must wait until the wrapped method has
completed before raising `CancelledError`s, otherwise the wrapped method
will continue running in the background with a logging context that has
been marked as finished.

Signed-off-by: Sean Quah <seanq@element.io>

Signed-off-by: Sean Quah <seanq@element.io>

…tion

Signed-off-by: Sean Quah <seanq@element.io>

Signed-off-by: Sean Quah <seanq@element.io>