SERVFAIL/timeout issue on iOS

[ameshkov]

Here's the problem with the current SERVFAIL handling:

1. We emulate SERVFAIL when we encounter a timeout exception (or any other exception)
2. This itself is a problem since iOS caches these SERVFAIL responses and never sends anything back
3. iOS (Android should do the same) re-sends the DNS query one more time when it itself encounters a timeout

We may need to reassess the way fallback works. Ideally, we should use the fallback when we receive the retransmitted request (it has the same ID, right?) and does nothing to the first request, simply let it hang for longer.

This behavior needs to be tested&verified on Windows, iOS, Mac, Android.

[ameshkov]

The issue does not seem to be fixed, let's do one more try.