load_hpack takes ~9% of total CPU at high request rates

[aryehlev]

Running an HTTP/2 server at ~1M req/s on ARM64, load_hpack is ~9% of total CPU in our profiles. Feels way too high for header decoding. I looked through the source and I think there's a lot of low-hanging fruit. Happy to send patches.

[seanmonstar]

Sure, optimizations are always welcome!