MINOR: mux-h2: make the initial window size configurable per side

MINOR: mux-h2: make the initial window size configurable per side

For a long time the initial window size (per-stream size) was set once
for both directions, frontend and backend, resulting in a tradeoff between
upload speed and download fairness. This commit allows it to be configured
separately for each side. The older settings remains the fallback choice
when other ones are not set.