zebra: separate netlink socket for dataplane

zebra: separate netlink socket for dataplane

Use a separate netlink socket for the dataplane's updates, to
avoid races between the dataplane pthread and the zebra main
pthread. Revise zebra shutdown so that the dataplane netlink
socket is cleaned-up later, after all shutdown-time dataplane
work has been done.

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