o Minor bugfixes:
    - Stop requiring exits to have non-zero bandwithcapacity in a
      TestingTorNetwork. Instead, when TestingMinExitFlagThreshold is 0,
      ignore exit bandwidthcapacity.
      This assists in bootstrapping a testing Tor network.
      Fixes bugs 13718 & 13839.
      Makes bug 13161's TestingDirAuthVoteExit non-essential.