Improve 'prevent-fetch' — allow adding arbitrary delay before returning response

A few sites started to exploit `performance.now` to see the timing difference with real ad requests. First spotted by @stephenhawk8054
on `https://kio.ac/c/ashM-l6lE_IvX2v9QxR5Gb` with following code
```
const i = e.now();
const s = await fetch(M(0), {
cache: M(4),
mode: M(3)
});
if (e.now() - i < 50) {
t(true);
return;
}
```

and now `pvpoke-re.com` too (skipping pasting the code as it's heavily obfuscated and long). uBO has addressed this by https://github.com/gorhill/uBlock/commit/d2bce26e7d537f156b6a5484efbba2ea0bbff24c