zebra: Add code to notice nexthop changes for pbr tables

zebra: Add code to notice nexthop changes for pbr tables

When we have a PBR installed as a table, we need to notice
when a nexthop changes and rethink the routes for the pbr
tables.

Add code to nexthop tracking to notice the pbr watched
nexthop has changed in some manner.  If it is a pbr route
that depends on the nexthop then just enqueue it for
rethinking.

This is a bit of a hammer, we know that only pbr routes
are going to be installing routes in weird non-standard
tables as such we need to only handle nexthop changes
for nexthops that are actually changing that we care
about and to only requeue for route nodes we have
route entries for from PBR

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>