nat: timed out session scavenging upgrade

Patch changes the behavior of session scavenging and fixes multiple
nat issues. Allows proper session clearing and removes issue with lingering sessions
in session db. Patch also updates and fixes CLI/API calls for better readability
of session state metrics. Fixes security issue that would allow attacker to
reuse timed out session in both directions (in2out/out2in).

Type: improvement

Signed-off-by: Filip Varga <fivarga@cisco.com>
Change-Id: I78897585a2a57291fad5db6d457941aa0a0457bd