o Major bugfixes:
    - When the exit relay gets a circuit-level sendme cell, it started
      reading on the exit streams, even if had 500 cells queued in our
      circuit queue already, so our circuit queue just grew and grew
      in some cases.  We fix this by not re-enabling reading on SENDME
      while the cell queue is blocked.  Fixes bug 1653.  Bugfix on
      0.2.0.1-alpha.  Detected by Mashael AlSabah.  Original patch by
      "yetonetime".