instead of keeping just random NS and address,
it keeps a map of nameservers => {addresslist}

this allows scoring and correct NS election, fallback
and evaluation

- cache is now fully optional, resolution works without cache as well
- proper mempool per each request, not mixed
- zone cut resolution doesn't need any per-context memory
- now it's possible to do qname minimization
- rplan keeps tabs on resolved queries
- added documentation etc.

Slow start evaluates all NS in the preference list, using mean as
a sorting function. Invalidated NSs are pushed to the back of the
preference list.