bgpd: Cleanup NHT state when underlying VRF goes down

bgpd: Cleanup NHT state when underlying VRF goes down

When the underlying VRF is deleted, ensure that state for the
next hops that BGP registers with zebra for tracking purposes is
properly updated. Otherwise BGP will not re-register the next hop
when the VRF is re-created, resulting in the next hop staying
unresolved.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Ticket: CM-17456
Reviewed By: CCR-6587
Testing Done: Manual, bgp-min, vrf