o Major bugfixes:
    - Newly created streams were allowed to read cells onto circuits,
      even if the circuit's cell queue was blocked and waiting to drain.
      This created potential unfairness, as older streams would be
      blocked, but newer streams would gladly fill the queue completely.
      We add code to detect this situation and prevent any stream from
      getting more than one free cell.  Bugfix on 0.2.0.1-alpha.
      Possible partial fix for bug 1298.