RIB: deduplicate RibManager::FaceIdSet and Rib::FaceLookupTable
RibManager maintains a FaceIdSet to detect missed face destroyed events.
Rib maintains a FaceLookupTable to delete affected routes when a face is destroyed.
These two data structures contain partially duplicate information, and should be combined.
Furthermore, FaceMonitor does not rely on RibManager's command processing and signature validation functions, so that it could be moved down to Rib class.