nat: TCP state tracking based on RFC 7857/RFC 6146

nat: TCP state tracking based on RFC 7857/RFC 6146

Implement proper state machine based on above RFCs. ACKs to SYNs/FINs
are no longer required/tracked. This is more friendly to peers and
accounts for lost packets and retransmits.

This change also means that all traffic is translated and forwarded
while in transitory timeout, which helps delivering e.g. retransmitted
FINs, FINACKs and other messages.

Also support reopening a session in transitory timeout after seeing both
FINs by seeing both SYNs again. This helps quick connection
reestablishment if the peers want to.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Signed-off-by: Miklos Tirpak <miklos.tirpak@gmail.com>
Change-Id: Ibf521c79463472db97e593bfa02b32b4a06dfd2a