PR cilium#25309 introduced a data race by sharing the sessionUDPFactory between the
DNS server instances for the different IP families (IPv4 & IPv6). This has been detected
by cilium#27979.

This commit fixes the data race by using dedicated instances of the sessionUDPFactory.

Fixes: cilium#28156

Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>

This commit replaces the global response UDP connections (IPv4 & IPv6)
variables with fields in the SessionUDPFactory. This way the connections
can already be setup during construction of the factory and the `sync.Once`s
(global vars) can be removed.

Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>

Historically, the `socketUDPFactory` and the lilsten configuration
supported IPv4 and IPv6 on the same socket.

Since the change to bind the DNS servers to localhost, a server
and socket is created per ip family.

Therefore this commit removes the support for multiple ipfamilies
on the same socket (SocketUDPFactory & listen configuration). This
improves readability quite a lot.

Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>

This commit adds the transparent socket options to the IPFamily
struct. This way it can be used in the respective functions.

Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>