Memory leak in Root

[kjvalencik]

Fixes a memory leak in Root reported in #746.

This PR includes two commits:

• Delete N-API references when the count hits 0.
  This is the bug referenced above. It stems from a misunderstanding of N-API. It's not sufficient to decrement the reference count to 0, the reference must also be deleted.
• Use an Option to store the reference instead of using ManuallyDrop.
  While not as extreme, the global event queue reference counter was always incrementing. It would never be freed, event after the context ended.

[dherman]

This looks great, it's just a little bit subtle and I don't 100% understand yet. How did the ManuallyDrop logic prevent the queue reference count from decrementing, and how does the Option fix that?

[kjvalencik]

This looks great, it's just a little bit subtle and I don't 100% understand yet. How did the ManuallyDrop logic prevent the queue reference count from decrementing, and how does the Option fix that?

@dherman Since Drop is recursive, ManuallyDrop prevents the fields from executing their Drop implementation as well. To drop the Arc would require some unsafe.

This changes the strategy so ManuallyDrop is no longer used. Instead, a None signals to the Drop implementation that nothing needs to be done. Since these fields are only used internally, the value will always be Some when a user interacts with it. The methods that take the value and replace it with None consume the struct. Since we control it, we can guarantee that only Drop will execute after the take.

[dherman]

Looks great!