Randomize the source port for each query in plain-UDP mode. Currently we
create a socket with net.ListenUDP and use it for all queries, which
means all queries have the same source address. ValdikSS reports that in
regions of Turkmenistan, UDP associations that use the same 4-tuple are
blocked after a few seconds.
https://ntc.party/t/topic/475
My idea for this is to create a new PacketConn abstraction that creates
a bunch (≈100) of goroutines that each do a single query at a time,
creating a new socket for each one and waiting for a response on the
same socket up to a timeout. Similar to HTTPPacketConn and its multiple
instantiations of sendLoop.
