o Minor features:
    - Path Use Bias: Perform separate accounting for successful circuit use.
      Separate statistics on stream attempt versus success rates are kept
      for each guard. Configurable thresholds are provided to determine
      when to emit log messages or disable use of guards that fail too
      many stream attempts.
  o Minor bugfixes:
    - Remove a source of rounding error during path bias count scaling.
    - Don't count cannibalized circuits as used for path bias until we
      actually try to use them.
    - Fix circuit_package_relay_cell warning message about n_chan==NULL.