MINOR: protocol: add get_src() and get_dst() at the protocol level

MINOR: protocol: add get_src() and get_dst() at the protocol level

Right now the proto_fam descriptor provides a family-specific
get_src() and get_dst() pair of calls to retrieve a socket's source
or destination address. However this only works for connected mode
sockets. QUIC provides its own stream protocol, which relies on a
datagram protocol underneath, so the get_src()/get_dst() at that
protocol's family will not work, and QUIC would need to provide its
own.

This patch implements get_src() and get_dst() at the protocol level
from a connection, and makes sure that conn_get_src()/conn_get_dst()
will automatically use them if defined before falling back to the
family's pair of functions.