Write and run a clique reachability test.

It would be useful to know what the full inter-relay connectivity graph looks like, and how far it differs from the "every relay can always reach every other relay" ideal.

https://www.sba-research.org/wp-content/uploads/publications/NavigaTor_preprint.pdf

This should be something doable with stem, and ideally we can run it periodically/automatically, and use it to do things like reject relays that have extremely poor connectivity.