fasthttp: process BSD requests outside the kqueue loop

[guweigang]

Summary

This changes the BSD fasthttp backend so complete HTTP requests are dispatched from the kqueue loop to worker threads, while connection close/rearm/write lifecycle operations are posted back to the event-loop thread through a command channel.

The BSD backend previously executed request_handler directly inside the kqueue loop. A handler that performed blocking work, including an HTTP request back to the same server, could prevent the loop from accepting or processing the nested request.

Details

• track connections that currently have a request being processed
• remove read/write interest before dispatching a complete request
• run the request handler asynchronously
• keep kqueue/client-map mutations on the event-loop thread via loop commands
• re-enable reads or writes from the loop after the worker finishes preparing the response
• preserve manual takeover and reusable takeover handling

This keeps each fd owned by only one active request at a time, while avoiding direct kqueue/client-map mutation from worker threads.

Tests

• v -cc cc -d use_openssl -enable-globals test vlib/fasthttp/fasthttp_test.v

The added regression test starts a fasthttp server and verifies that a handler for /outer can synchronously fetch /inner from the same server.

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