MINOR: backend: switch to conn_get_{src,dst}() for port and address mapping

MINOR: backend: switch to conn_get_{src,dst}() for port and address mapping

The backend connect code uses conn_get_{from,to}_addr to forward addresses
in transparent mode and to map server ports, without really checking if the
operation succeeds. In preparation of future changes, let's switch to
conn_get_{src,dst}() and integrate status check for possible failures.