lib/zebra: convert vrf_list to a red-black tree

lib/zebra: convert vrf_list to a red-black tree

Since we're already using a red-black tree to store VRFs sorted by their
vrf_id's, create a new tree to store VRFs sorted by their names.

The biggest advantage of doing this is that we reduce the time complexity
of vrf_list_lookup_by_name() from O(n) to O(log n).

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