Fix QNetworkReply on WASM

There are multiple problems with sending request and QNetworkReply on WebAssembly platform:

1. The request can only be send from main thread. There was a fix which proxied request to main thread but it was reverted as it introduced another problems.
2. Cancelling the request is done by setting m_fetch->userData to nullptr. This can be done better by calling emscripten_fetch_close().
3. Request is not cancel when QNetworkReply is deleted. This can lead to use-after-free by emscripten fetch. Doublecheck the lifetimes of objects passed to emscripten_fetch().

Related: QTBUG-122893