Wire HttpParser to ASGI hot path, replacing AsyncRequest.parse() with
direct buffer-based parsing. Add FastAsyncRequest wrapper for body
reading. Replace per-request Queue/Task with BodyReceiver for on-demand
body reading. Keep headers as bytes end-to-end to avoid conversion
overhead. Add backpressure control and keepalive timer. Cache response
status lines and Date header.
Benchmark shows 3x improvement: ~875K req/s for simple GET (was ~340K).
- Read chunk size lines and trailers in 64-byte blocks instead of 1 byte
at a time, pushing back excess data to the unreader buffer
- Reuse BytesIO buffers with truncate/seek instead of creating new
objects to reduce GC pressure in AsyncUnreader
- Use bytearray.find() directly instead of converting to bytes first
in header parsing loop
- Use index-based iteration for header parsing instead of list.pop(0)
which is O(n) per pop vs O(1) for index access
Add tests for the optimized parsing code paths.
