zebra: add flag track released state of proto NHGS

zebra: add flag track released state of proto NHGS

Add a flag to track the released state of a proto-based NHG.

This flag is used to know whether the upper level proto has called
the *_del API. Typically, the NHG would just get removed and uninstalled
at this point but there is a chance we are being sent it while routes
are still being owned or we were sent it multiple times. This flag
and associated code handles that.

Ticket: CM-30369

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