lib: Switch to flow based tunneling.

lib: Switch to flow based tunneling.

With this patch, ovs-vswitchd uses flow based tunneling
exclusively.  I.E. each kind of tunnel shares a single tunnel
backer in the datapath.  Tunnel headers are set by userspace using
the ipv4_tunnel datapath action.  And, the configuration of
individual tunnels is now a userspace responsibility, so
netdev-vport no longer marshals and unmarshals Netlink attributes
for tunnel configuration, instead only storing the configuration
internally.  There are still some significant pieces of work to do,
but the basic building blocks are there to begin testing.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Co-authored-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>