tor/changes/bug9063_redux

  o Major bugfixes:
    - When we have too much memory queued in circuits (according to a new
      MaxMemInCellQueues option), close the circuits consuming the most
      memory.  This prevents us from running out of memory as a relay if
      circuits fill up faster than they can be drained.  Fixes
      bug 9063; bugfix on the 54th commit of Tor. This bug is a further
      fix beyond bug 6252, whose fix was merged into 0.2.3.21-rc.

      Also fixes an earlier approach taken in 0.2.4.13-alpha, where we
      tried to solve this issue simply by imposing an upper limit on the
      number of queued cells for a single circuit.  That approach proved to
      be problematic, since there are ways to provoke clients to send a
      number of cells in excess of any such reasonable limit.
      Fixes bug 9072; bugfix on 0.2.4.13-alpha.