MINOR: session: Add src and dst addresses to the session

MINOR: session: Add src and dst addresses to the session

For now, these addresses are never set. But the idea is to be able to set
client source and destination addresses at the session level without
updating the connection ones.

Functions to fill these addresses have been added: sess_get_src() and
sess_get_dst(). If not already set, these functions relies on
conn_get_src() and conn_get_dst() to fill session addresses.

And just like for conncetions, sess_src() and sess_dst() may be used to get
source and destination addresses. However, if not set, the corresponding
address from the underlying client connection is returned. When this
happens, the addresses is filled in the connection object.