Work out why Tor networks with all-zero bandwidths are unstable
When I run a chutney network with an empty bwfile, the network seems to fail more often.
We should either:
- fix this bug on tor clients, or
- make authorities never vote zero bandwidths:
- make the minimum bandwidth 1, or
- drop relays with zero bandwidths from the consensus.
This blocks adding a bwfile test to make test-network-all in #26803 (moved)