session: improve session lookup speeds

session: improve session lookup speeds

It would seem that this avoids using the stack for computing the
subsequent crc32 by storing the 2 x u64 ip4 5-tuple key into two
registers. Probably the cast to the "convenience" data structure "masks"
the fact that the key is 16B. Probably ... :-)

"sh run" on an ip4 tcp sender reports that the number of clocks spent in
tcp_input, where a session lookup is performed, drops ~25% (from ~100
clocks/packet to ~75 clocks/packet)

Change-Id: I50d647115ac72f8407bff7e3b73328ba42f0ddb5
Signed-off-by: Florin Coras <fcoras@cisco.com>