previously cryptolib random function was used to generate message id,
this works well but it is slow especially when the entropy is low,
replaced with cryptographically safe prng ISAAC

the ccan directory is going to be used in the future, as it's include
structure makes it easy to embed C snippets instead of reimplementing
them

refs #15

* each query is assigned a task
* each task contains request, some primitives and mempool
* worker can process multiple tasks at once and
  offload I/O to event loop

Not finished:

* it depends on icmp/system timeouts, #22
* tcp reads are going to be bad if the messages
  arrive fragmented #21

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

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

this fixes resolution of multi-level queries, when a side query
encounters a nameserver that requires address resolution