MEDIUM: backend: turn all conn->addr.{from,to} to conn->{src,dst}

MEDIUM: backend: turn all conn->addr.{from,to} to conn->{src,dst}

All reads were carefully reviewed for only reading already checked
values. Assignments were commented indicating that an allocation will
be needed once they become dynamic. The memset() used to clear the
addresses should then be turned to a free() and a NULL assignment.