Fixing memory leak that leaves values and callbacks in memory indefinitely

[nosachamos]

Keeping track of return values that arrive before callbacks are registered, and then delivering to the callbacks when they are registered.

This fixes a major memory leak currently in Eel, as reported in #540

Tested extensively locally.

See that values no longer accumulate when many rapid calls to JS are performed:

[nosachamos]

@ChrisKnott this is an important one... may warrant a new release, perhaps?

[ChrisKnott]

@ChrisKnott this is an important one... may warrant a new release, perhaps?

Yes, I think so. One thing I never really thought about was that people would make apps with Eel that run for hours.

I have had a lot of Life Stuff last couple of years but from this week I will have Friday afternoons free so hopefully I can finally get back to making progress on this project. It's kind of got away from me atm 😅...

[samuelhwilliams]

Hey @nosachamos - this looks like it'd be really good for us to get in. I'm aware it's been a long time since you contributed this - sorry.

If you still have context, do you think you'd be able to add a test that can demonstrate the memory leak is fixed (and will therefore protect against regressions in the future)