Emscripten fetch API won't invoke callback if the calling thread is in waiting.

We ran into an issue with Emscripten fetch API in ASYNC mode.
After a thread issued a Emscripten fetch call, we put it in waiting status, meaning we call std::conditional_variable::wait() on the calling thread. In this case, the fetch callbacks never get invoked.

The structure of our code is something like:

- function definition:

```
void invokeNetworkingCall(callback){
_attr.onsuccess = callback;
emscripten_fetch(_attr, ...);
}
```
/////////////////////////////////////////////

- In the calling thread:
```

std::conditional_variable c;
invokeNetworkingCall([&c]{
// some operations
c.notify_all();
});
c.wait();
```

I've read the documentation about the "Waitable" mode, but not sure if that's related to this issue. But anyway, I wonder what's the reason this happens, even I've pass the -pthread + -s PROXY_TO_PTHREAD to the compiler/linker, which enable pthread and ran app on it. Shouldn't the callback of "Fetch API" triggered from a different thread than the calling one?

Thanks!

Inizia dall’entry point emscripten_fetch e dal comportamento di ASYNC, Waitable, -pthread e PROXY_TO_PTHREAD descritto nel report. Riproduci il fallimento del callback con std::condition_variable::wait(), quindi traccia come vengono pianificati i fetch callbacks e controlla i test esistenti relativi al threading o al fetch. Il lavoro è completato quando il comportamento documentato o previsto del callback è stato stabilito e un test di regressione copre il caso del thread in attesa.

Scritto dal modello di indicizzazione a partire dal testo della issue.