o Major features (consensus downloads):
    - Schedule multiple in-progress consensus downloads during client
      bootstrap. Use the first one that starts downloading, close the
      rest. This reduces failures when authorities are slow or down.
      With #15775, it reduces failures due to fallback churn.
      Implements #4483 (reduce failures when authorities are down).
      Patch by "teor".
      Implements IPv4 portions of proposal #210 by "mikeperry" and
      "teor".