bgpd: Reset LLA NHT's interface if there is a change

bgpd: Reset LLA NHT's interface if there is a change

For link-local IPv6 next hops, the next hop tracking is implemented based
on interface status changes. For this purpose, the ifindex is stored in
the NHT. Reset this value if a change in ifindex is noticed, such as for
example after a restart of the networking service.

Also add some additional debug logs.

Signed-off-by: Vivek Venkatraman <vivek@nvidia.com>
Updates: "bgpd: Switch LL nexthop tracking to be interface based"

Ticket: RM 2575386
Testing Done:
1. Manual verification
2. Precommit (#156), evpn-smoke (#155), bgp-smoke (#157), vrl (#158)
-- Precommit is clean, reported failures in evpn-smoke & vrl are resolved
-- some other tests fail in evpn-smoke, bgp-smoke & vrl, appear to be existing
-- or unrelated failures