There are two big changes here:
  - We store active circuits in a priority queue for each or_conn,
    rather than doing a linear search over all the active circuits
    before we send each cell.
  - Rather than multiplying every circuit's cell-ewma by a decay
    factor every time we send a cell (thus normalizing the value of a
    current cell to 1.0 and a past cell to alpha^t), we instead
    only scale down the cell-ewma every tick (ten seconds atm),
    normalizing so that a cell sent at the start of the tick has
    value 1.0).