dpdk: Userspace Datapath: Introduce NAT Support.

dpdk: Userspace Datapath: Introduce NAT Support.

This patch introduces NAT support for the userspace datapath.
Most conntrack module changes are in this patch, with the
exception of icmp related handling and recent orig tuple
support.

The per packet scope of lookups for NAT and un_NAT is at
the bucket level rather than global. One hash table is
introduced to support create/delete handling. The create/delete
events may be further optimized, if the need becomes clear.

Some NAT options with limited utility (persistent, random) are
not supported yet, but will be supported in a later patch.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Daniele Di Proietto <diproiettod@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>