zebra: Add handling for kernel del/update nexthop

zebra: Add handling for kernel del/update nexthop

Add handling for delete/update nexthop object messages from the
kernel.

If someone deletes a nexthop object we are still using, send it back
down. If the someone updates a nexthop we are using, replace that nexthop
with ours. Routes are referencing this nexthop object ID and we resolved
it ourselves, so we should force the other `someone` to submit to our
will.

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