staticd: refcount the nht add/removal

staticd: refcount the nht add/removal

When we add / remove a nexthop that we need to track,
keep track of the number of times we have done this
for each nexthop.  Consequently keep track of the
number of available nexthops, so that we can
just install new routes when we get one
that uses a pre-existing nexthop.  Deletion of
nexthops is done on refcount going to 0.
Removal of routes is handled elsewhere for removal.

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