Implement flow-control from proposal 324

This ticket, split from #88, is to implement implement the stream-based flow-control parts of proposal 324. They are:

• Flow control proper.
  • Likely refactoring of stream window and stream buffering code to handle multiple behaviors (40h)
  • Implement handling for incoming XON/XOFF (16h)
  • Keep an EWMA drain rate estimator for each stream (24h)
  • Send XON/XOFF as appropriate based on EWMA estimator, and sidechannel mitigation (24h)
  • Experiment and revise to determine correct XON/XOFF thresholds. (40h)
• Support for a per-TCP-stream token-bucket rate-limiting implementation (est 16h)