nat: tweak rfc7857 tcp connection tracking

nat: tweak rfc7857 tcp connection tracking

The RFC7857 state machine introduced in 56c492a is a trade-off.
It tries to retain sessions as much as possible and also offers
some protection against spurious RST by re-establishing sessions if data
is received after the RST. From experience in the wild, this algorithm is
a little too liberal, as it leaves too many spurious established sessions
in the session table.

E.g. a oberserved pattern is:
client      server
         <- FIN, ACK
ACK      ->
ACK      ->
RST, ACK ->

With the current state machine this would leave the session in established state.

These proposed changes do:
 - require 3-way handshake to establish session.
   (current requires only to see SYNs from both sides)
 - RST will move session to transitory without recovery if data is sent after
 - Only a single FIN is needed to move to transitory

Fixes: 56c492aa0502751de2dd9d890096a82c5f04776d
Type: fix
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: I92e593e00b2efe48d04997642d85bd59e0eaa2ea
Signed-off-by: Ole Troan <ot@cisco.com>