o Major bugfixes (bufferevents):
    - Apply rate-limiting only at the bottom of a chain of filtering
      bufferevents. This prevents us from filling up internal read
      buffers and violating rate-limits when filtering bufferevents
      are enabled. Bugfix on 0.2.3.1-alpha; fixes part of bug 3804.
    - Add high-watermarks to the output buffers for filtered
      bufferevents. This prevents us from filling up internal write
      buffers and wasting CPU cycles when filtering bufferevents are
      enabled.  Bugfix on 0.2.3.1-alpha; fixes part of bug 3804.