nat: more long read after short write optimization

Replace whitespread (mis)use of snat_session_key_t by proper function
arguments where applicable and inline functions to calculate hash keys
instead of using structs for that. Make all hash tables use same network
byte order port so that there is no longer a discrepancy between static
mappings using host byte order while in2out/out2in tables using network
byte order.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I80786d2f947c67824c101a13bb608f1fe1080f34