OPTIM: quic: reduce the size of struct quic_dgram

OPTIM: quic: reduce the size of struct quic_dgram

The QUIC code can only handle IPv4 or IPv6 addresses, so using two
sockaddr_storage structs wastes a lot of space in the quic_dgram struct.
This is a very large overhead since this structure is written in the MPSC
ring buffers before every datagram, while many of those datagrams are only
50 bytes or less. Using an union instead saves 200 bytes per datagram,
increasing the capacity of the buffers significantly.