Fix lambda cross-thread dynamics (take 2)

[RandomShaper]

This PR has two commits:

1. Reverts recent attempts to fix a chain of issues, which was making the whole thing extremely complex.
2. Implements an alternative to Fix lambda cross-thread dynamics (reverted) #84659 (the first reverted one), much simpler, now I've been lucid enough to realize how to do it properly. I'm muuuch more satisfied with this approach. (Reviewers are advised to check the diff of just this one, to be back to the starting point.)

Supersedes #85223.
Fixes #85151.

Tested the following recent related bugs are still gone after this PR (plus no address sanitizer issues detected);

• Deferred call of GDScript lambda from thread leads to double free or corruption when hot reloading #84046
• Lambda call within a non-main thread forces Godot to crash at exit #84190
• Lambda/Callable memory leak #85112

[YuriSizov]

I left a couple of questions, but overall it feels good and this does look simpler. I can almost understand what's going on 🙃

I tested #85151 and #85112, which I was able to reproduce before, and can confirm this PR fixes/refixes them.

[akien-mga]

Tested successfully on a few projects using lambdas.

[akien-mga]

Thanks!