Dynamic Window Sizing

[Stebalien]

Currently, yamux uses a fixed-size window. Unfortunately, this means we can't account for variable latencies. For low-latency connections, the window is too large and we'll have to hold large buffers. For high-latency connections, the window is too small and we'll bottleneck on filling up the window.

Ideally, we'd extract the QUIC dynamic window sizing logic from https://github.com/lucas-clemente/quic-go and use it to figure out our window sizing.