Refactor async code

[sebastianpoeplau]

This is an attempt at making the code for asynchronous fuzzing easier to understand.

Feel free to reject the PR if you think it's not clearer than what we started with. The need to work around the issues we found with Napi::AsyncWorker (i.e., losing stack traces) and Napi::Error (i.e., inability to copy it outside the main thread) have made the code a bit less readable than I had hoped.