this allows list-like declaration of bound interfaces

e.g. net = { ‘127.0.0.1’, net.eth0 }

the cache can be setup declaratively with:

cache.size = 10 * MB

the cache storage backend can be changed with:

cache.storage = ‘lmdb://.'

module api can now store userdata, e.g. owner

the TCP allowed parallel processing of multiple requests over
one client socket, however if the client socket disconnected,
it left the running tasks reading from bad handle
now each task takes ownership of the handle until it is finished,
only then it is returned to the loop and closed

the pktcache introduces two cache tags PKT/SEC for
basic/DNSSEC responses and stores negative answers in
the cache

previously producers in overlay could only generate query or
update the final answer - this required exported callbacks
from iterate.c and specific processing. this wouldn’t work
for negative cache, as it would be required to reimplement
whole iterate layer for cached data

the new workflow allows produce layers to generate answer
instead, this answer is then consumed in the next step
by the iterate module in unified fashion for all caches/generators

this fixes stripped DNSSEC records in final answer

the server responds with edns0 if the client requests it,
it also uses DNSSEC for queries if DO=1

the obtained records are not however validated

the engine keeps an array of storage backends available
for caching, it is possible to switch the backend on
runtime with standard ‘proto://config' string

the default backend is ‘lmdb://<path>’

the api is still a global variable, but it would be the
best if the namedb_t carried the pointer to the api,
so it needs to change in libknot later

each asset is tagged by a byte defining its type and importance
this is a groundwork for negative cache and packet cache
abstracted the code in preparation for different backends