zebra: handle gracefulRS/retain with proto NHGs

zebra: handle gracefulRS/retain with proto NHGs

Properly handle refcounting of Proto-owned NHGs when
zebra is operating under graceful restart and retain
conditions.

We have an extra refcnt of 1 we keep for proto-owned NHGs to
indicate the upper level proto has created and owns it.

When we are reading these in from the kernel, we need to set them
to 1 as appropriate. Without this, we fail in the assert() during
zebra_nhg_proto_add() after the owning daemons resends the NHG
and the refcnts are off by one.

Also add in the same logic we use for routes when sweeping with
respect to uptimes.

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