Reduce congestion window after idle period

[martinthomson]

When we don't send for a little while, it might be a good idea to have the congestion window decay a little. Otherwise, we might start up sending at the previous rate, which might no longer be valid.

One approach might be to count the number of elapsed PTOs and reduce exponentially based on that number, down to the initial congestion window.

(Note: idle period, not idle timeout.)