Prevent tokio runtime from being dropped outside of our control

[Wodann]

#771 highlighted the fact that the tokio::Runtime can be dropped outside of our control, even if we have tokio::runtime::Handles that haven't been dropped yet. This resulted in unexpected behaviour when calling synchronisation primitives in our own Drop implementations.

To avoid this, we should ensure that we ourselves can determine when the async runtime is dropped.

This is blocked by a re-design of our async architecture, as we currently are using napi's tokio::Runtime.