[mDNS]Initial commit for RR caching
Working mDNS cache + fixed cookie lookup

- cookie lookup also matching qtype now
[mDNS] Improved RR caching

- Don't cache RR when TTL = 0
- Schedule RR deletion when in cache and TTL = 0

Known issue: IPv6 IPs in cache are incorrect
Merge branch 'master' of github.com:tass-belgium/picotcp