when doing fast retransmit, each address may be
contacted 1-N times, but previously only cumulative
RTT was tracked for the NS that sent the answer.

now the approximate query start time is subtracted
from cumulative RTT to give an idea how long it
took since the query was actually sent, and all
the NSs that didn't respond within their retransmit
windows are penalised too

bcf3f4b5 made query_finalize() to be called on
each outbound query before sending, so each
retransmitted query got different 0x20 case
and the only responses from the latest contacted
NS matched.

reverted back the old behaviour where query case
is randomised when an outbound query is generated,
not before sending

REFUSED response no longer causes retry in
iterator when operating in stub mode

example:

> daf.add 'forward 127.0.0.1@5353'

EDNS data from the packet must be erased before attempting to send the
query again.

the cookies require libknot>=2.3, Makefile didn't
check for that, and turning cookies off wasn't
possible either

Introduced kr_rplan_push_empty() to create empty query for DNS cookies.