nat: nat44-ed add session timing out indicator in api

nat: nat44-ed add session timing out indicator in api

Type: improvement

Currently, NAT44-ED users sessions details are returned for both active
and timed out NAT sessions. It may confuse users that expect to see only
active sessions in the response and make them think that timeouts for
NAT sessions do not work.

With this change, introduce an indicator of timing out for NAT sessions
returned in NAT44-ED user session details.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: Ib81ed90369d4b495117538f521e5112e289cf7e6