* Reduced some SimpleTimer churn
* add hooks for per-peer choking in the outbound message queue - if/when a
  peer reaches their cwin, no further messages will enter the 'active' pool
  until there are more bytes available.  other messages waiting (either later
  on in the same priority queue, or in the queues for other priorities) may
  take that slot.
* when we have a message acked, release the acked size to the congestion
  window (duh), rather than waiting for the second to expire and refill the
  capacity.
* send packets in a volley explicitly, waiting until we can allocate the full
  cwin size for that message