lib: implement NS reputation, write address election code

Problem

if a NS has an unreachable address, resolver discards the whole nameserver record. Then the nameserver record is fetched again and random server is tried leading to the same error. This leads to short resolution loops for broken servers.

Example

0.gravatar.com A
ns1.edgecastcdn.net. has broken AAAA records, leading to resolution fail

Expected behaviour

if an address is unreachable, it should be marked as this in the reputation system. On the next iteration, address election should be ran and different address should be selected.

• Zone cut keeps a set of addresses to choose from (instead of just one)
  • Needs a different structure to avoid frequent realloc (pack)
  • Zone cut needs a map of NS names to records
• Rework iterator to use address set
• Best address election function
• Keep elected address in the kr_query (as sockaddr)
• Prefer IPv6 if possible (bonus score)
• Fuzzy selection (message id may be used as a random value)

Reassigned to @mvavrusa

Status changed to closed

In 2fff8c8c

mentioned in issue #283 (closed)