WIP: [hibernated] client-subnet
Mainly cache changes for now.
Status: all deckard tests succeed (non-ECS).
It was decided to deviate from rfc7871, as full implementation would be potentially very cache-hungry (there are very many meaningful IP prefixes). Instead, a geo DB will be used to partition IPs into discrete categories. That's what the authoritative NSs use AFAIK.
For now the granularity would be a country, as used by https://www.maxmind.com/en/geoip2-databases
Cache deduplicates equal records for different locations. That's implemented by splitting the mapping into two: usual key + location -> hash, and usual key + hash -> RRdata. Timestamp and TTL of the RRset is stored in the first part, and the stored RRs have TTLs zeroed. (All this forced changes in lib/cache.h API.)