the reason is that LMDB allows only 1 write transaction
per thread, and this doesn’t work well with multiplexed
requests. there is probably going to be some performance hit,
but now transactions are scoped for each cache operation

the worker now creates a resolution context copy,
and keeps it if the query requires iterative queries.
the worker_exec() is now a reentrant function that gets
called with incoming data until the resolution is done,
and it sends the answer

TODO: per-loop freelists are implemented

* updated documentation
* the props and config api has some workarounds yet

if the module declares such properties, they can be accessed
by treating module as a table, e.g.

hints[‘localhost’] = ‘127.0.0.1’
hints.hostname = ‘192.168.1.1.
print(hints.localhost)

equals to:
hints.set(‘localhost 127.0.0.1’)
hints.set(‘hostname 192.168.1.1’)
print(hints.get(‘localhost’))

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).