[HTTP/2] Investigate requests latency

[c410-f3r]

https://github.com/LesnyRumcajs/grpc_bench showed that wtx generally performs well but some requests, specially in the 99% area, end up having higher latencies.

I have a hunch that a such thing happens because Http2::accept needs to wait for a stream buffer provided by the user instead of creating one as soon a header frame is received. Or maybe not, there needs to be more investigation/profiling.

• Add the wtx project LesnyRumcajs/grpc_bench#473 (comment)
• 2024-09-20 i5-6600K (4) @ 3.90 GHz, 16GB LesnyRumcajs/grpc_bench#475