nat: ED: global session LRU list

nat: ED: global session LRU list

Maintain a global session LRU allowing reuse of expired session instead
of relying on a scavenging mechanism to periodically walk sessions.
Whenever a new session is being allocated in slow path, also attempt to
free an expired session from global LRU list.

Type: improvement

Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I9edde9ec138de67c9a4888e915b0490ec16415fa