Send DNS A and AAAA queries in parallel

Send DNS A and AAAA queries in parallel

This implements sending DNS A & AAAA queries in parallel by
creating "slave" idns_query requests. Current implementation
uses the A lookup as "master" and AAAA as "slave" query.

Long term this should probably be restructured to separate "lookup
state" and "query", or even better yet to defer the DNS lookups
until connect time and perform A respective AAAA as needed only and
not look up both before attemting to connect.

This also drops the dns_v4_fallback directive as it have no effect with
parallel DNS lookups. This directive should be reinstanciated in future
to enable pure IPv6 usage.