if the query is satisfied from cache, it doesn’t need a zone cut lookup, so it may be deferred until an outbound query is about the be issued.

this prepares cache/txn structures to hold API as well, so we can get
rid of the global api

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

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

if the server doesn’t understand edns0, replies with
notimpl, formerr or sends bad badly formed response,
we try basic dns query (ns0.rbsov.bbc.co.uk)

the encloser might exist and contain valid data, the resolver would return wrong records in this case

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