zebra: Parse in nexthop ID information from new routes

zebra: Parse in nexthop ID information from new routes

Add parsing code for nexthop object ID's when we get a
route. When we get a new route with the new kernel, it
will come with a nexthop ID and the nexthop full info.
We should just reference by ID if it exists and point
to the nexthop hash entry that matches it.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>