Rate Limiting, fixes #35

Fixes #35 
Good point, I remember now that in Unblock IPv4 and IPv6 didn't share limiters, I'll simply add the protocol to the key. I'm very aware that large networks share the same public IPs. But the limiter values here are *shorter than the time to blink your eyes*. And on top of that there's a queue (max length of 2). The reason it works in practice is because bots do bursts of simultaneous requests, while humans don't (in the case of humans they're not *perfectly* simultaneous). Believe me, it looks really harsh, but it works really well in practice.

I'm willing to be convinced to make them configurable, but I don't think it's a good idea and here's why. No matter where you put your server, if it's accessible from the internet, there'll be bots. Those bots scan all of the IPv4 space to find openly accessible DNS servers, so EVERYBODY that hosts a DNS server gets the same bots. Therefore, it's our job to tweak those values to find the sweet spot and those default values will be good for everybody. Adding them to the config makes everything more complicated. Adding them to the config makes it possible to REALLY break DNSChain and tweaking them requires a very good understanding of concurrency and experience administrating DNS servers. Therefore, I'm against it, but if you have better reasons than mine I'm always willing to be convinced.

The penalty is only something that affects BLOCK mode, so that's why I only set it in that one specific case.