o Minor bugfixes:
    - Never relay a cell for a circuit we have already destroyed.
      Between marking a circuit as closeable and finally closing it,
      it may have been possible for a few queued cells to get relayed,
      even though they would have been immediately dropped by the next
      OR in the circuit.  Fix 1184; bugfix on 0.2.0.1-alpha.
    - Never queue a cell for a circuit that's already been marked
      for close.