h2-repro

This reproduces hyper issue #2419.

cargo run --release is enough to show the problem: making n POST requests through hyper::Client, where n is more than the maximum allowed number of concurrent streams, results in requests hanging:

$ cargo run --release --quiet
2022-10-20T20:24:33.711722Z  INFO h2_repro: 100 requests on 50 streams
2022-10-20T20:24:33.711745Z  INFO h2_repro: (Set $H2_REQUESTS and $H2_MAX_STREAMS environment variables to adjust)
2022-10-20T20:24:33.843178Z  INFO h2_repro: H1: Completed 100 / 100
2022-10-20T20:24:34.436008Z ERROR h2_repro: H2: Stuck at 51 / 100

A more complete write-up is available at https://fasterthanli.me/articles/the-http-crash-course-nobody-asked-for#bugs-bugs-bugs