download: optional prefer ipv6 record

[myfreeer]

In package cacheable-lookup, the lookup method would lookup for both ipv4 and ipv6 records, but it would put put ipv4 records before ipv6 records if the dns resolver returns both results. The method query would pick the first result to return, which in this case would be ipv4 result.
Setting family to 6 for cacheable-lookup or setting dnsLookupIpVersion to ipv6 would not help since there is still many mdn domain with only ipv4 address.
For users with faster ipv6 network than ipv4, this can make downloading faster for developer.mozilla.org or any future ipv6 hosts.