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 legacy/trac#26803 (moved)