validator can now yield, but it doesn't plan the sub-requests directly,
that is still a job of the driver

this caters a use case when a layer needs to issue subrequests before
continuing, so it yields. when the subrequests finish, the layer is
resumed with the same parameters and input

todo: fix validator, that shifted most of the processing to driver

this is useful when you need to issue several subrequests before
continuing with the current query, resuming is not supported yet, so it
will requery after the subrequests complete

current processed query is always in `request->current_query`

when no validation is attempted, trust level is 'bad'
when validation attempted but insecure, trust level is 'insecure'
otherwise 'secure'

minimised nodata/nxdomain can now be cached if it is
authoritative. also pkt/sec cache are now shared using
the ‘rank’ field to replace insecure version with secure
if needed

this is a workaround for missing DEFER operation, as the
validator module can only detect trust chain breakage
(caused by answering from different authority) after the
iterator writes answer. this causes duplicated answer on
uncached queries

this doesn’t fix record duplication in answer when not answered from cache

missing boundary checking and bad offset calculation

todo:
the code presumes little endian, didn’t get to  fix that

if the query has RD=0 or is ANY, only cache is probed
for ANY, only A/AAAA/MX is checked and no query is
forwarded to the authoritatives

effectively enables/disables usage of given IP protocol
for subrequests (the server can still listen on these)

if the answer is flagged as insecure, it means that the resolver tried
to validate it, but couldn't (e.g. trust chain doesn't exist)

the key is now stored in a format friendly to prefix search, the values
also contain one more 16bit field to store rank of the data (to be
utilised later)

this is a problem when both CNAME and the target are answered from the same NS (but different authority), but only the CNAME authority does DNSSEC. it’s probably legal, but it’s pretty stupid to do so

before the algorithm was happy with root hints for all queries starting
at root, however they're often overloaded and result in timeouts
the updated code provides SBELT only for root NS query lookup and tries
to use cached information as much as possible

previously it was always overwritten with SBELT for root + root TA
doesn't have to be in cache (it's in trust store)

refs #33