Fib algo: extend KPI by allowing algo to set datapath pointers.

Some algorithms may require updating datapath and control plane
algo pointers after the (batched) updates.

Export fib_set_datapath_ptr() to allow setting the new datapath
function or data pointer from the algo.
Add fib_set_algo_ptr() to allow updating algo control plane
pointer from the algo.
Add fib_epoch_call() epoch(9) wrapper to simplify freeing old
datapath state.

Reviewed by: zec
Differential Revision: https://reviews.freebsd.org/D29799
MFC after: 1 week