Initial implementation of RRL.

Based on memo and implementation notes from
Vixie and Schryver.
http://ss.vix.su/~vixie/isc-tn-2012-1.txt

Basically a token bucket algorithm, no interpolation
yet. Classification of responses based on:
<address prefix, resp.class, name, seed>

address prefix = /24 for IPv4, /56 for IPv6
resp.class = based on rcode,question and ancount
name = either qname or answer
seed = secret to harden collision prediction

No SLIP yet.