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).
Closes#3484
When a client disconnects during an ASGI request, the worker now:
1. Sends http.disconnect message to the app's receive queue
2. Allows a configurable grace period for cleanup (default: 3 seconds)
3. Only cancels the task after the grace period expires
This follows the ASGI HTTP Connection Scope spec which defines
http.disconnect as the message apps should receive when clients
disconnect: https://asgi.readthedocs.io/en/latest/specs/www.html#disconnect-receive-event
The grace period prevents CancelledError from propagating to async
database operations, allowing SQLAlchemy and other async DB libraries
to properly reset their connection pools.
New config option: --asgi-disconnect-grace-period (default: 3 seconds)
