ldpd: fix processing of redistributed routes

ldpd: fix processing of redistributed routes

Commit 5048fe changed the way zebra behave when a route is updated. Now,
whenever a route is changed, zebra advertises its new version without
withdrawing the old one. This patch adapts ldpd to understand this new
behavior. After processing a ZEBRA_REDISTRIBUTE_IPV[46]_ADD message,
we need to check for nexthops that were removed and, for each of them
(if any), withdraw the associated labels from zebra.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>