nameservers with addresses are preferred, the rest
has same weight

no blacklisting, penalization or rtt scoring yet

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

On FreeBSD the lua package was not detected. (Tested on 9.3 and 10.1).

extracted part about writing layers to lib documentation