zebra: start dataplane layer work

zebra: start dataplane layer work

Reduce or eliminate use of global zebra_ns structs in
a couple of netlink/kernel code paths, so that those paths
can potentially be made asynch eventually.

Slide netlink_talk_info into place to remove dependency on core
zebra structs; add accessors for dplane context block

Start init of route context from zebra core re and rn structs;
start queueing and event handling for incoming route updates.

Expose netlink apis that don't rely on zebra core structs;
add parallel route-update code path using the dplane ctx;
simplest possible event loop to process queued route'
updates.

Signed-off-by: Mark Stapp <mjs@voltanet.io>