zebra: Sweep our nexthop objects out on restart

zebra: Sweep our nexthop objects out on restart

On restart, if we failed to remove any nexthop objects due
to a kill -9 or such event, sweep them if we aren't using them.
Add a proto field to handle this and remove the is_kernel bool.

Add a dupicate flag that indicates this nexthop group is only
present in our ID hashtable. It is a dupicate nexthop we received
from the kernel, therefore we cannot hash on it.

Make the idcounter globally accessible so that kernel updates
increment it as soon as we receive them, not when we handle them.

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