Allow max_concurrent_reset_streams>0 on browser environments

[Mubelotix]

I'm willing to set max_concurrent_reset_streams to a value higher than 0. In browser environments, this causes h2 to panic (see #511).

I'm willing to work on this issue. I could add a web feature enabling the optional instant dependency and being used instead of std::time::Instant to provide time. #511 was closed by a temporary fix.