lib: parallel queries
Some queries can be made in parallel (A+AAAA).
rplan can only work with the current query at the top of the stack.
The change would be to store a pointer to
current that would be chosen when the
answer comes based on following criteria:
msgid + <qname, qtype, qclass> match,
and the query MUST NOT have a parent. This could be later used for look-ahead queries (DNSKEY),
but then a care must be taken as the answers MAY come out of order, while they MUST be processed in order.