Try to improve the fallback approach

### Issue Details

First of all, why do we need a fallback? We faced issues with nameservers for unknown reasons preventing access from our name servers. There were not too many, but still this is a pretty problematic issue for AdGuard DNS users.

Currently, we do the following: if the recursor returns a `SERVFAIL` response, AG DNS tries to use a fallback.

Unfortunately, this leads to issues like that: https://github.com/AdguardTeam/AdGuardDNS/issues/653

### Proposed solution

1. The simple solution would be to use the fallback only once per eTLD+1 domain. If the fallback also returns a `SERVFAIL` response for that domain name, remember this and don't use the fallback anymore.
2. Disable fallbacks for specific domain zones.

### Alternative solution

If we had a pure go recursor implementation we could come up with a more sophisticated solution, but we don't.