Mux egress rate limitting for Leios traffic

[coot]

Rate limiting for egress traffic.

Some open questions:

• can this be implemented so it self adopts to network conditions, rather than a static configuration?

[karknu]

I wonder if we could use delay monitoring and implement a ledbat-like scheme for Leios traffic.
We track the base/min delay per connection over a window.
We use token bucket to control Leois sending rate with a dynamic fill rate.
When the increased delay is below a given target we increase the fill rate.
When the increased delay is above we decrease the fill rate.