packets through that point have been ACKed, throwing an
  InterruptedIOException if there was a writeTimeout or an IOException
  if the con failed
* revamped the ack/nack field settings to ack as much as possible
* handle some strange timeout/resend errors on connection
* pass 1/2rtt as the packet 'optional delay' field, and use that to
  schedule the ack time (the 'last' messages in a window set the
  optional delay to 0, asking for immediate ack of all received)
* increase the optional delay to 2 bytes (#ms to delay)
* inject random failures and delays if configured to do so in
  PacketHandler.choke
* fix up the window size adjustment (increment on ack, /= 2 on resend)
* use the highest RTT in the new RTT calculation so that we fit more
  in (via SACK)
* fix up the SACK handling (duh)
* revise the resend time calculation