Move rate-limiting from per-client to per-client-per-domain

[DL6ER]

What does this implement/fix?

Let me be clear here: Rate-limit is there it protect you. It is not meant as a method to cause pain. What are we protecting against? Basically against Denial of Service (DoS) "attacks" either by a malicious attacker or by some client that has a bug or crashed or ... (you never know, we have seen so may things). If one attacker or or misbehaving client can take down your entire network because your DNS resolver runs out of memory - that doesn't sound like something you'd want, right? - and this is where FTL's rate-limiting kicks in, simply discarding queries when there are too many. The default settings are suitable both for home and enterprise, small and huge networks, however, they can always be adjusted (or even disabled) to your liking if you feel you need no DoS-protection.

This PR proposes to change FTL's rate-limiting mechanism from a pure "per-client" to a "per-client and per-domain" scheme. If a client queries domain.com like hell, it will get rate-limited as before. However, any other domain (say example.com) can still resolved (except, obviously, it it gets queried too often, too).

This reduces the "there is not Internet connection" effect some users reported at the costs of a much less effective rate-limiting as clients and/or malicious attackers can simply evade it by querying varying domains. We have seen this by buggy clients querying seemingly random subdomain - each of them would be counted on their own.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): to be done

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[DL6ER]

Rebased on latest development

[yubiuser]

Why do we keep this here? RATE_LIMIT_MESSAGE is not used anymore.

[yubiuser]

Suggested change

	// Get child's DNS cache entry (we eed to create a new cache entry here
	// Get child's DNS cache entry (we need to create a new cache entry here

[yubiuser]

Still need L208 "RATE_LIMIT_MESSAGE"?

[DL6ER]

The unique change we added recently won't work here as uniqueness is now composed out of three fields for this new message type (type + client + domain). Needs more work.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Existing merge conflicts have not been addressed. This PR is considered abandoned.