route: Ensure nexthops are reregistered correctly

Reregister prefixes if nexthops don't change
Remove invalid nexthops instead of all of them
Cancel FibEntry expiration events before scheduling new ones
Remove FibEntry refreshing
Add grace period to LSA expiration

refs: #1892

Change-Id: I5136b6b0f52a47b6ba72e04b0cea81b85ebccd45