Changes since 3.5.2:
1）3.5 replaced our ANY query with A+AAAA for users with IPv6 enabled. Extensive measurements by Darren Gamble showed that this change had a non-trivial performance impact. We now do the ANY query like before, but fall back to the individual A+AAAA queries when necessary. Change in commit 1147a8b.
2）The IPv6 address for d.root-servers.net was added in commit 66cf384, thanks Ralf van der Enden.
3）We now drop packets with a non-zero opcode (i.e. special packets like DNS UPDATE) earlier on. If the experimental pdns-distributes-queries flag is enabled, this fix avoids a crash. Normal setups were never susceptible to this crash. Code in commit 35bc40d, closes ticket 945.