Convergence Improvements

Convergence Improvements

addressing convergence times when interface is shut.
1) prioritise the registered callback handlers. Add FIB convergence handler as high priority
2) hook the FIB convergence call-back into HW link down.
3) don't schedule a walk of a FIB node if it has no children
4) Checks at fib_path_t to prevent unnecessary walks, that it prevent the same information propagting the graph multiple times.

Change-Id: I406966b50f31d77c221821b8649776d66655194c
Signed-off-by: Neale Ranns <nranns@cisco.com>