Add parameter to cap cwnd during slow start
After discussing queue overload with @dgoulet based on some analysis of trying to minimize circ_max_cell_queue_size
for congestion control, and reviewing old simulator results, I realized that slow-start overshoot is still a component cause of large queues. In shadow, the max cwnd after 4MB for normal circuits was ~4000 cells, but slow start maxed out as high as 20000 cells.
Unfortunately, our slow start increment (50%) is already pretty low. I think a better fix rather than lowering it further is to provide a parameter to exit slow start at some max (say 5000 or 10000). Then, steady state can ramp up or down depending on spare capacity after that max, without as much risk of overshoot.
I can write this and test it in shadow, and see if it helps reduce large queue sizes there without an impact on performance.