beaconing: select more segments in propagation/registration
beaconing: select more segments in propagation/registration

The low default value of the DefaultBestSetSize meant that
even small core networks might not have the desired path
diversity. This is especially true when ASes had multiple
links between them, as most of the set would be used by
the multiple links.

GitOrigin-RevId: c91f7a591a24dd802ce3df25ff74f828771ccf02

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion