Fix a leak in `FutureState` when a `Notifier` is dropped un-woken

Fix a leak in `FutureState` when a `Notifier` is dropped un-woken

If a `Notifier` has an internal `FutureState` which gathers some
sleeper callbacks, but is never actaully woken, those callbacks
will leak due to a circular `Arc` reference when the `Notifier` is
`drop`'d.

Because `Notifier`s are rarely `drop`'d in production this isn't a
huge deal, but shows up materially in bindings tests as they spawn
many nodes over the course of a short test.

Fixes #2232