Provide a smarter way to detect & block DNS amplification

First of all, this feature is important only to those who run AGH as a public DNS server.

Currently, we just ship AGH with a relatively small rate limit value by default. This is enough to guarantee that we don't do much harm, but the server stats are distorted anyway.

It'd be better if AGH could automatically block domain names used for amplification.

I suggest using a rather simple algorithm for now:
1. If the client is being constantly rate-limited
2. Most of this client's DNS queries are for the same domain name
3. We should add this domain name to "Disallowed domains" automatically

Users should be able to disable this behavior.

Add a new setting to "DNS settings -> Access settings":

* Name: "Smart DNS amplification detection"
* Description: "Automatically detect and block domain names that are being used for DNS amplification attacks. Please note, that this feature makes sense only if you run AdGuard Home on a public server."