Rate Limiting, fixes #35

Fixes #35 
I disagree with you.

Each DNS lookup is 1000 times bigger than the limiter it happens in. All the objects involved in Native-dns and the like are HUGE. The DNS response needs to be constructed and all the buffers, strings and concatenations involved are orders of magnitude bigger than a limiter (that is shared between requests anyway). It makes no sense to optimize the 0.001%, especially when it adds a lot of complexity to the code. And even if all the rest of the DNS code was optimized, it would still be 100+ times bigger than the limiter and even if it was on a massive server, it would still die from CPU exhaustion or run out of network bandwidth before objects less than a KB in size matter.