fabricd: build a list of neighbors and neighbors neighbors

fabricd: build a list of neighbors and neighbors neighbors

OpenFabric uses a list of neighbors and neighbors neighbors to calculate
a set of designated reflooders.

While the draft prescribes that these lists should be built whenever an
LSP needs to be flooded, this implementation opted to build them only
when we ran an spf, given that they will only change when the topology
changes.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>